Chapter V: Part 5

The _Cerebellum_, LITTLE BRAIN, or AFTER BRAIN occupies the inferior
pair of occipital fossae, and lies below the plane of the tentorium
cerebelli. It consists of two hemispheres or lateral lobes, and of a
median or central lobe, which in human anatomy is called the vermis.
It is connected below with the medulla oblongata by the two restiform
bodies which form its _inferior peduncles_, and above with the corpora
quadrigemina of the cerebrum by two bands, which form its _superior
peduncles_; whilst the two hemispheres are connected together by the
transverse fibres of the pons, which form the _middle peduncles_ of
the cerebellum. On the superior or tentorial surface of the cerebellum
the median or vermiform lobe is a mere elevation, but on its inferior
or occipital surface this lobe forms a well-defined process, which
lies at the bottom of a deep fossa or _vallecula_; this fossa is
prolonged to the posterior border of the cerebellum, and forms there a
deep notch which separates the two hemispheres from each other; in
this notch the falx cerebelli is lodged. Extending horizontally
backwards from the middle cerebellar peduncle, along the outer border
of each hemisphere is the _great horizontal fissure_, which divides
the hemisphere into its tentorial and occipital surfaces. Each of
these surfaces is again subdivided by fissures into smaller lobes, of
which the most important are the _amygdala_ or _tonsil_, which forms
the lateral boundary of the anterior part of the vallecula, and the
_flocculus_, which is situated immediately behind the middle peduncle
of the cerebellum. The inferior vermiform process is subdivided into a
posterior part or _pyramid_; an elevation or _uvula_, situated between
the two tonsils; and an anterior pointed process or _nodule_.
Stretching between the two flocculi, and attached midway to the sides
of the nodule, is a thin, white, semilunar-shaped plate of nervous
matter, called the inferior _medullary velum_.

The whole outer surface of the cerebellum possesses a characteristic
foliated or laminated appearance, due to its subdivision into
multitudes of thin plates or lamellae by numerous fissures. The
cerebellum consists of both grey and white matter. The grey matter
forms the exterior or cortex of the lamellae, and passes from one to
the other across the bottoms of the several fissures. The white matter
lies in the interior of the organ, and extends into the core of each
lamella. When a vertical section is made through the organ, the
prolongations of white matter branching off into the interior of the
several lamellae give to the section an arborescent appearance, known
by the fanciful name of _arbor vitae_ (see fig. 6). Independent masses
of grey matter are, however, found in the interior of the cerebellum.
If the hemisphere be cut through a little to the outer side of the
median lobe, a zigzag arrangement of grey matter, similar in
appearance and structure to the nucleus of the olivary body in the
medulla oblongata, and known as the _corpus dentatum_ of the
cerebellum, is seen; it lies in the midst of the white core of the
hemisphere, and encloses white fibres, which leave the interior of the
corpus at its inner and lower side. On the mesial side of this _corpus
dentatum_ lie three smaller nuclei. The white matter is more abundant
in the hemispheres than in the median lobe, and is for the most part
directly continuous with the fibres of the peduncles of the
cerebellum. Thus the restiform or inferior peduncles pass from below
upward through the white core, to end in the grey matter of the
tentorial surface of the cerebellum, more especially in that of the
central lobe; on their way they are connected with the grey matter of
the corpus dentatum. The superior peduncles, which descend from the
corpora quadrigemina of the cerebrum, form connexions mainly with the
corpus dentatum. The middle peduncles form a large proportion of the
white core, and their fibres terminate in the grey matter of the
foliated cortex of the hemispheres. It has been noticed that those
fibres which are lowest in the pons go to the upper surface of the
cerebellum and vice versa.

_Histology of the Cerebellum._--The white centre of the cerebellum is
composed of numbers of medullated nerve fibres coursing to and from
the grey matter of the cortex. These fibres are supported in a
groundwork of neuroglial tissue, their nutrition being supplied by a
small number of blood vessels.

FIG. 7.--Transverse Section through a Cerebellar Folium (after
Kolliker). Treated by the Golgi method.

P. Axon of cell of Purkinje.
F. Moss fibres.
K and K^1. Fibres from white core of folium ending in molecular
layer in connexion with the dendrites of the cells of Purkinje.
M. Small cell of the molecular layer
GR. Granule cell.
GR^1. Axons of granule cells in molecular layer cut transversely.
M^1. Basket-cells.
ZK. Basket-work around the cells of Purkinje.
GL. Neuroglial cell.
N. Axon of an association cell.]

The cortex (see fig. 7) consists of a thin layer of grey material
forming an outer coat of somewhat varying thickness over the whole
external surface of the laminae of the organ. When examined
microscopically it is found to be made up of two layers, an outer
"molecular" and an inner "granular" layer. Forming a layer lying at
the junction of these two are a number of cells, the _cells of
Purkinje_, which constitute the most characteristic feature of the
cerebellum. The bodies of these cells are pear-shaped. Their inner
ends taper and finally end in a nerve fibre which may be traced into
the white centre. In their course through the granule layer they give
off a number of branching collaterals, some turning back and passing
between the cells of Purkinje into the molecular layer. Their inner
ends terminate in one or sometimes two stout processes which
repeatedly branch dichotomously, thus forming a very elaborate dendron
in the molecular layer. The branchings of this dendron are also highly
characteristic in that they are approximately restricted to a single
plane like an espalier fruit tree, and those for neighbouring cells
are all parallel to one another and at right angles to the general
direction of the folium to which they belong. In the molecular layer
are found two types of cells. The most abundant are the so-called
_basket cells_ which are distributed through the whole thickness of
the layer. They have a rounded body giving off many branching dendrons
to their immediate neighbourhood and one long neuraxon which runs
parallel to the surface and to the long axis of the lamina. In its
course, this gives off numerous collaterals which run downward to the
bodies of Purkinje's cells. Their terminal branchings together with
similar terminals of other collaterals form the basket-work around the
bodies of these cells.

The granular layer is sometimes termed the rust-coloured layer from
its appearance to the naked eye. It contains two types of nerve cells,
the small granule cells and the large granule cells. The former are
the more numerous. They give off a number of short dendrites with
claw-like endings, and a fine non-medullated neuraxon process. This
runs upward to the cortex, where it divides into two branches in the
form of a T. The branches run for some distance parallel to the axis
of the folium and terminate in unbranched ends. The large granule
cells are multipolar cells, many of the branchings penetrating well
into the molecular layer. The neuraxon process turns into the opposite
direction and forms a richly branching system through the entire
thickness of the granular layer. There is also an abundant plexus of
fine medullated fibres within the granule layer.

The fibres of the white central matter are partly centrifugal, the
neuraxons of the cells of Purkinje, and partly centripetal. The
position of the cells of these latter fibres is not known. The fibres
give rise to an abundant plexus of fibrils in the granular layer, and
many reaching into the molecular layer ramify there, especially in the
immediate neighbourhood of the dendrites of Purkinje's cells. From the
appearance of their plexus of fibrils these are sometimes called _moss
fibres_.

The _Fourth Ventricle_ is the dilated upper end of the central canal
of the medulla oblongata. Its shape is like an heraldic lozenge. Its
floor is formed by the grey matter of the posterior surfaces of the
medulla oblongata and pons, already described (see figs. 3 and 6); its
roof partly by the inferior vermis of the cerebellum, the _nodule_ of
which projects into its cavity, and partly by a thin layer, called
_valve of Vieussens_, or superior _medullary velum_; its lower lateral
boundaries by the divergent clavae and restiform bodies; its upper
lateral boundaries by the superior peduncles of the cerebellum. The
_inferior medullary velum_, a reflection of the pia mater and
epithelium from the back of the medulla to the inferior vermis, closes
it in below. Above, it communicates with the _aqueduct of Sylvius_,
which is tunnelled below the substance of the corpora quadrigemina.
Along the centre of the floor is the median furrow, which terminates
below in a pen-shaped form, the so-called _calamus scriptorius._
Situated on its floor are the fasciculi teretes, striae acusticae, and
deposits of grey matter described in connexion with the medulla
oblongata. Its epithelial lining is continuous with that of the
central canal.

_The Cerebrum._

The _Cerebrum_ or GREAT BRAIN lies above the plane of the tentorium,
and forms much the largest division of the encephalon. It is customary
in human anatomy to include under the name of cerebrum, not only the
convolutions, the corpora striata, and the optic thalami, developed in
the anterior cerebral vesicle, but also the corpora quadrigemina and
crura cerebri developed in the mesencephalon or middle cerebral
vesicle. The cerebrum is ovoid in shape, and presents superiorly,
anteriorly and posteriorly a deep _median longitudinal fissure_, which
subdivides it into two hemispheres. Inferiorly there is a continuity
of structure between the two hemispheres across the mesial plane, and
if the two hemispheres be drawn asunder by opening out the
longitudinal fissure, a broad white band, the _corpus callosum_, may
be seen at the bottom of the fissure passing across the mesial plane
from one hemisphere to the other. The outer surface of each hemisphere
is convex, and adapted in shape to the concavity of the inner table of
the cranial bones; its inner surface, which bounds the longitudinal
fissure, is flat and is separated from the opposite hemisphere by the
falx cerebri; its under surface, where it rests on the tentorium, is
concave, and is separated by that membrane from the cerebellum and
pons. From the front of the pons two strong white bands, the _crura
cerebri_ or _cerebral peduncles_, pass forward and upward (see fig.
2). Winding round the outer side of each crus is a flat white band,
the _optic tract_. These tracts converge in front, and join to form
the _optic commissure_, from which the two _optic nerves_ arise. The
crura cerebri, optic tracts, and optic commissure enclose a
lozenge-shaped space, which includes--(a) a grey layer, which, from
being perforated by several small arteries, is called _locus
perforatus posticus_; (b) two white mammillae, the _corpora
albicantia_; (c) a grey nodule, the _tuber cinereum_, from which (d)
the _infundibulum_ projects to join the _pituitary body_. Immediately
in front of the optic commissure is a grey layer, the _lamina cinerea_
of the third ventricle; and between the optic commissure and the inner
end of each Sylvian fissure is a grey spot perforated by small
arteries, the _locus perforatus anticus_.

If a transverse section is made at right angles to the surface of the
crura cerebri it will pass right through the mesencephalon and come
out on the dorsal side through the corpora quadrigemina (see fig. 8).
The ventral part of each crus forms the crusta, which is the
continuation forward of the anterior pyramidal fibres of the medulla
and pons, and is the great motor path from the brain to the cord.
Dorsal to this is a layer of pigmented grey matter, called the
_substantia nigra_, and dorsal to this again is the tegmentum, which
is a continuation upward of the formatio reticularis of the medulla,
and passing through it are seen three important nerve bundles. The
superior cerebellar peduncle is the most internal of these and
decussates with its fellow of the opposite side so that the two
tegmenta are continuous across the middle line. More externally the
mesial fillet is seen, while dorsal to the cerebellar peduncle is the
posterior longitudinal bundle. If the section happens to pass through
the superior corpus quadrigeminum a characteristic circular area
appears between the cerebellar peduncle and the fillet, which, from
its tint, is called the red nucleus. More dorsally still the section
will pass through the Sylvian aqueduct or passage from the third to
the fourth ventricle, and this is surrounded by a mass of grey matter
in the ventral part of which are the nuclei of the third and fourth
nerves. The third nerve is seen at the level of the superior corpus
quadrigeminum running from its nucleus of origin, through the red
nucleus, to a groove on the inner side of the crus called the
_oculo-motor_ groove, which marks the separation between the crusta
and tegmentum. Dorsal to the Sylvian aqueduct is a layer called the
_lamina quadrigemina_ and on this the corpora quadrigemina rest. The
superior pair of these bodies is overlapped by the pineal body and
forms part of the lower visual centres. Connexions can be traced to
the optic tract, the higher visual centre on the mesial surface of the
occipital lobe, the deep origin of the third or oculo-motor nerve as
well as to the mesial and lateral fillet. The inferior pair of
quadrigeminal bodies are more closely in touch with the organs of
hearing, and are connected by the lateral fillet with the cochlear
nucleus of the auditory nerve.

FIG. 8.--Transverse Section through the Human Mesencephalon at the
level of the superior Quadrigeminal Body.]

_Surface of the Brain._

The peripheral part of each hemisphere, which consists of grey matter,
exhibits a characteristic folded appearance, known as gyri (or
convolutions) of the cerebrum. These gyri are separated from each
other by _fissures_ and _sulci_, some of which are considered to
subdivide the hemisphere into lobes, whilst others separate the gyri
in each lobe from each other. In each hemisphere of the human brain
five lobes are recognized: the temporo-sphenoidal, frontal, parietal,
occipital, and the central lobe or Island of Reil; it should, however,
be realized that these lobes do not exactly correspond to the outlines
of the bones after which they are named. Passing obliquely on the
outer face of the hemisphere from before, upward and backward, is the
well marked _Sylvian fissure_ (fig. 9, s), which is the first to
appear in the development of the hemisphere. Below it lies the
temporo-sphenoidal lobe, and above and in front of it, the parietal
and frontal lobes. As soon as it appears on the external surface of
the brain the fissure divides into three limbs, anterior horizontal
(s^1), ascending (s^2), and posterior horizontal (s^3), the latter
being by far the longest. The place whence these diverge is the
Sylvian point and corresponds to the pterion on the surface of the
skull (see ANATOMY: _Superficial and Artistic_). Between these three
limbs and the vallecula or main stem of the fissure are four
triangular tongues or opercula; these are named, according to their
position, orbital (fig. 9, C), frontal (pars triangularis) (B),
fronto-parietal (pars basilaris) (A) and temporal. The frontal lobe is
separated from the parietal by the _fissure of Rolando_ (fig. 9, r)
which extends on the outer face of the hemisphere from the
longitudinal fissure obliquely downward and forward towards the
Sylvian fissure. About 2 in. from the hinder end of the hemisphere is
the _parieto-occipital fissure_, which, commencing at the longitudinal
fissure, passes down the inner surface of the hemisphere, and
transversely outwards for a short distance on the outer surface of the
hemisphere; it separates the parietal and occipital lobes from each
other.

FIG. 9.--Gyri and Sulci, on the outer surface of the Cerebral
Hemisphere.

f^1, Sulcus frontalis superior.
f^2, Sulcus frontalis inferior.
f.m, Sulcus frontalis medius.
p.m, Sulcus paramedialis.
A, Pars basilaris.
B, Pars triangularis.
C, Pars orbitalis.
S, Sylvian fissure.
s^1, Anterior horizontal limb (Sylvian fissure).
s^2, Ascending limb (Sylvian fissure).
s^3. Posterior horizontal limb (Sylvian fissure).
s.asc, Ascending terminal part of the posterior horizontal limb of
the Sylvianfissure.
p.c.i, Inferior praecentral sulcus.
p.c.s, Superior praecentral sulcus.
r, Fissure of Rolando.
g.s, Superior genu.
g.i, Inferior genu.
d, Sulcus diagonalis.
t^1, Superior temporal sulcus (parallel sulcus).
t^2, Inferior temporal sulcus.
p^1, Inferior postcentral sulcus.
p^2, Superior postcentral sulcus.
p^3, Ramus horizontalis.
p^4, Ramus occipitalis.
s.o.t, Sulcus occipitalis transversus.
occ. lat, Sulcus occipitalis lateralis (the sulcus lunatus of Elliot
Smith).
c.m, Calloso-marginal sulcus.
c.t.r, Inferior transverse furrow.]

The _Temporo-Sphenoidal Lobe_ presents on the outer surface of the
hemisphere three convolutions, arranged in parallel _tiers_ from above
downward, and named _superior, middle and inferior temporal_ gyri. The
fissure which separates the superior and middle of these convolutions
is called the _parallel fissure_ (fig. 9, t^1). The _Occipital Lobe_
also consists from above downwards of three parallel gyri, named
_superior, middle and inferior occipital_. The _Frontal Lobe_ is more
complex; immediately in front of the fissure of Rolando, and forming
indeed its anterior boundary, is a convolution named _ascending
frontal_ or pre-central, which ascends obliquely backward and upward
from the Sylvian to the longitudinal fissure. Springing from the front
of this gyrus, and passing forward to the anterior end of the
cerebrum, are three gyri, arranged in parallel _tiers_ from above
downwards, and named _superior, middle and inferior frontal_ gyri,
which are also prolonged on to the orbital face of the frontal lobe.
The _Parietal Lobe_ is also complex; its most anterior gyrus, named
_ascending parietal_ or post-central, ascends parallel to and
immediately behind the fissure of Rolando. Springing from the upper
end of the back of this gyrus is the supra-parietal lobule, which,
forming the boundary of the longitudinal fissure, extends as far back
as the parieto-occipital fissure; springing from the lower end of the
back of this gyrus is the _supra-marginal_, which forms the upper
boundary of the hinder part of the Sylvian fissure; as this gyrus
occupies the hollow in the parietal bone, which corresponds to the
eminence, it may appropriately be named the _gyrus_ of the _parietal
eminence_. Above and behind the gyrus of the parietal eminence is the
_angular gyrus_, which bends round the posterior extremity of the
parallel fissure, while arching over the hinder end of the inferior
temporo-sphenoidal sulcus is the post-parietal gyrus. Lying in the
parietal lobe is the _intra-parietal_ fissure (fig. 9, p^3 and p^4),
which separates the gyrus of the parietal eminence from the
supra-parietal lobule.

The _Central Lobe_ of the hemisphere, more usually called the _insula_
or _island of Reil_, does not come to the surface of the hemisphere,
but lies deeply within the Sylvian fissure, the opercula forming the
margin of which, conceal it. It consists of four or five short gyri,
which radiate from the _locus perforatus anticus_, situated at the
inner end of the fissure. This lobe is almost entirely surrounded by a
deep sulcus called the limiting sulcus of Reil, which insulates it
from the adjacent gyri. It lies opposite the upper part of the
ali-sphenoid, where it articulates with the parietal and
squamous-temporal.

17. Convolution of the margin of the longitudinal fissure.
O. Olfactory fissure, over which the olfactory peduncle and lobe are
situated.
TR. Orbital sulcus.
1" 1"'. Convolutions on the orbital suface.
1,1,1,1. Under surface of infero-frontal convolution.
4. Under surface of ascending frontal; and 5, of ascending parietal
convolutions.
C. Central lobe or insula.]

In front of the central lobe, on the base of the brain, are the
_orbital gyri_, which are separated from one another by the _orbital
sulcus_. This is usually H-shaped, and the gyri are therefore
anterior, posterior, external and internal. Bisecting the internal
orbital gyrus is an antero-posteripr sulcus (_s. rectus_), beneath
which lies the olfactory lobe, bulbous in front, for the olfactory
nerves to arise from.

On the mesial surface of the hemisphere, as seen when the brain is
longitudinally bisected and the cerebellum and medulla removed by
cutting through the crus cerebri (see fig. 11), the divided corpus
callosum is the most central object, while below it are seen the
fornix, septum lucidum and third ventricle, the description of which
will follow. The cerebral surface, above and in front of the corpus
callosum, is divided into two by a sulcus, the contour of which
closely resembles that of the upper margin of the corpus callosum.
This is the _calloso-marginal sulcus_, so called because it separates
the callosal gyrus, which lies between it and the corpus callosum,
from the marginal gyri nearer the margin of the brain. When the sulcus
reaches a point vertically above the hind end of the corpus callosum
it turns sharply upward and so forms the hinder limit of the marginal
gyri, the posterior inch or two of which is more or less distinctly
marked off to form the _paracentral lobule_, where the upper part of
the central fissure of Rolando turns over the margin of the brain. The
callosal gyrus, which is also called the gyrus fornicatus from its
arched appearance, is continued backward round the posterior end of
the corpus callosum, and so to the mesial surface of the temporal
lobe. Behind the upturned end of the calloso-marginal sulcus there is
a square area which is called the _precuneus_ or _quadrate lobe_; it
is bounded behind by the deeply cut internal parieto-occipital fissure
and this runs from the margin of the brain downward and forward to
join another fissure, the calcarine, at an acute angle, thus enclosing
a wedge-shaped piece of brain called the _cuneus_ between them. The
_calcarine_ fissure is fairly horizontal, and is joined about its
middle by the internal parieto-occipital, so that the part in front
of the junction is called the _pre-calcarine_, and that behind the
_post-calcarine_ fissure. The internal parieto-occipital and calcarine
are real fissures, because they cause an elevation in the interior of
the brain, known as the hippocampus minor. Just in front of the
anterior end of the calcarine fissure the callosal gyrus is
constricted to form the isthmus which connects it with the hippocampal
or uncinate gyrus. Below the calcarine fissure is a gyrus called the
_gyrus lingualis_, and this is bounded below by another true fissure,
the _collateral_, which runs parallel to the calcarine, but is
continued much farther forward into the temporal lobe and so forms the
lower boundary of the hippocampal gyrus. It will thus be seen that the
hippocampal gyrus is continuous posteriorly with the callosal gyrus
above by means of the isthmus, and with the gyrus lingualis below. The
hippocampal gyrus is bounded above by the dentate or hippocampal
fissure which causes the hippocampus major in the descending cornu and
so is a complete fissure. If its lips are separated the fascia dentata
or gyrus dentatus and the fimbria continued from the posterior pillar
of the fornix are seen. Anteriorly the fissure is arrested by the
recurved process of the upper part of the hippocampal gyrus, called
the _uncus_, and in front of this a slight sulcus, the _incisura
temporalis_, marks off the temporal pole or tip of the temporal lobe
from the region of the uncus. It will be seen that the callosal gyrus,
isthmus, and hippocampal gyrus form nearly a complete ring, and to
this the name of _limbic lobe_ is given.

_Interior of the Cerebrum._

If a horizontal slice be removed from the upper part of each
hemisphere (see fig. 12), the peripheral grey matter of the gyri will
be seen to follow their various windings, whilst the core of each
gyrus consists of white matter continuous with a mass of white matter
in the interior of the hemisphere. If a deeper slice be now made down
to the plane of the corpus callosum, the white matter of that
structure will be seen to be continuous with the white centre of each
hemisphere known as the centrum ovale. The _corpus callosum_ does not
equal the hemispheres in length, but approaches nearer to their
anterior than their posterior ends. It terminates behind in a free
rounded end, named the splenium (see fig. 11), whilst in front it
forms a knee-shaped bend, and passes downwards and backwards as far as
the lamina cinerea. If the dissection be performed on a brain which
has been hardened in spirit, the corpus callosum is seen to consist
almost entirely of bundles of nerve fibres, passing transversely
across the mesial plane between the two hemispheres; these fibres may
be traced into the white cores and grey matter of the gyri, and
connect the gyri, though by no means always corresponding ones, in the
opposite hemispheres. Hence the corpus callosum is a connecting or
commissural structure, which brings the gyri of the two hemispheres
into anatomical and physiological relation with each other. On the
surface of the corpus callosum a few fibres, the _striae
longitudinales_, run in the antero-posterior or longitudinal direction
(see fig. 12, b). Their morphological interest is referred to in the
section below on _Comparative Anatomy_. In the sulcus between the
corpus callosum and the limbic lobe a narrow band of fibres called the
_cingulum_ is seen, most of its fibres only run a short distance in it
and link together adjacent parts of the brain. If the corpus callosum
be now cut through on each side of its mesial line, the large cavity
or _lateral ventricle_ in each hemisphere will be opened into.

FIG. 11.--The Gyri and Sulci on the Mesial Aspect of the Cerebral
Hemisphere, r, Fissure of Rolando. r, o, Rostral sulcus. i, t,
Incisura temporalis.]

The lateral ventricle is subdivided into a _central space_ or body,
and three bent prolongations or _cornua_; the _anterior cornu_ extends
forward, outward and downward into the frontal lobe; the _posterior
cornu_ curves backward, outward and inward into the occipital lobe;
the _descending cornu_ curves backward, outward, downward, forward
and inward, behind and below the optic thalamus into the
temporo-sphenoidal lobe. On the floor of the central space may be seen
from before backward the grey upper surface of the pear-shaped caudate
nucleus of the _corpus striatum_ (figs. 12 and 13, f), and to its
inner and posterior part a small portion of the _optic thalamus_,
whilst between the two is the curved flat band, the _taenia
semicircularis_ (figs. 12 and 13, g). Resting on the upper surface of
the thalamus is the vascular fringe of the velum interpositum, named
_choroid plexus_, and immediately internal to this fringe is the free
edge of the white _posterior pillar of the fornix_. The anterior cornu
has the anterior end of the corpus striatum projecting into it. The
posterior cornu has an elevation on its floor, the _hippocampus minor_
(fig. 12, n), and between this cornu and the descending cornu is the
elevation called _eminentia collateralis_, formed by the collateral
fissure (fig. 12, o).

a, Transverse fibres, and
b, Longitudinal fibres of corpus callosum.
c, Anterior, and
d, Posterior cornua of lateral ventricle.
e, Septum lucidum.
f, Corpus striatum.
g, Taenia semicircularis.
h, Optic thalamus.
k, Choroid plexus.
l, Taenia hippocampi.
m, Hippocampus major.
n, Hippocampus minor.
o, Eminentia collateralis.]

Extending down the descending cornu and following its curvature is the
_hippocampus major_, which terminates below in a nodular end, the _pes
hippocampi_; on its inner border is the white _taenia hippocampi_,
continuous above with the posterior pillar of the fornix. If the
taenia be drawn to one side the hippocampal fissure is exposed, at the
bottom of which the grey matter of the gyrus hippocampi may be seen to
form a well-defined dentated border (the so-called _fascia dentala_).
The choroid plexus of the pia mater turns round the gyrus hippocampi,
and enters the descending cornu through the lateral part of the great
transverse fissure between the taenia hippocampi and optic thalamus.
The lateral ventricle is lined by a ciliated epithelium called the
_ependyma._ This lining is continuous through the foramen of Monro
with that of the third ventricle, which again is continuous with the
lining of the fourth ventricle through the aqueduct of Sylvius. A
little fluid is contained in the cerebral ventricles, which, under
some pathological conditions, may increase greatly in quantity, so as
to occasion considerable dilatation of the ventricular cavities.

If the corpus callosum be now divided about its middle by a transverse
incision, and the posterior half of this structure be turned back (see
fig. 13), the body of the fornix on which the corpus callosum rests is
exposed. If the anterior half of the corpus callosum be now turned
forward, the grey partition, or _septum lucidum_, between the two
lateral ventricles is exposed. This septum fits into the interval
between the under surface of the corpus callosum and the upper surface
of the anterior part of the fornix. It consists of two layers of grey
matter, between which is a narrow vertical mesial space, the _fifth
ventricle_ (fig. 13, e), and this space does not communicate with the
other ventricles nor is it lined with ependyma. If the septum be now
removed, the anterior part of the fornix is brought into view.

The _fornix_ is an arch-shaped band of nerve fibres extending in the
antero-posterior direction. Its anterior end forms the _anterior_
pillars of the arch, its posterior end the _posterior pillars_, whilst
the intermediate _body_ of the fornix forms the crown of the arch. It
consists of two lateral halves, one belonging to each hemisphere. At
the summit of the arch the two lateral halves are joined to form the
_body_; but in front the two halves separate from each other, and form
two anterior pillars, which descend in front of the third ventricle to
the base of the cerebrum, where they form the _corpora albicantia_,
and from these some white fibres called the bundle of Vicq d'Azyr
ascend to the optic thalamus (see fig. 11). Behind the body the two
halves diverge much more from each other, and form the posterior
pillars, in the triangular interval between which is a thin lamina of
commissural fibres called the _lyra_ (fig. 13, a). Each posterior
pillar curves downward and outward into the descending cornu of the
ventricle, and, under the name of _taenia hippocampi_, forms the
mesial free border of the hippocampus major (fig. 13, l). Eventually
it ends in the substance of the hippocampus and in the uncus of the
temporal lobe. If the body of the fornix be now divided by a
transverse incision, its anterior part thrown forward, and its
posterior part backward, the great transverse fissure of the cerebrum
is opened into, and the velum interpositum lying in that fissure is
exposed.

The _velum interpositum_ is an expanded fold of pia mater, which
passes into the anterior of the hemispheres through the great
transverse fissure. It is triangular in shape; its base is a line with
the posterior end of the corpus callosum, where it is continuous with
the external pia mater; its lateral margins are fringed by the choroid
plexuses, which are seen in the bodies and descending cornua of the
lateral ventricles, where they are invested by the endothelial lining
of those cavities. Its apex, where the two choroid plexuses blend with
each other, lies just behind the anterior pillars of the fornix. The
interval between the apex and these pillars is the aperture of
communication between the two lateral ventricles and the third,
already referred to as the foramen of Monro. The choroid plexuses
contain the small _choroidal arteries_; and the blood from these is
returned by small veins, which join to form the _veins of Galen._
These veins pass along the centre of the velum, and, as is shown in
fig. 1, open into the straight sinus. If the velum interpositum be now
carefully raised from before backward, the optic thalami, third
ventricle, pineal body and corpora quadrigemina are exposed.

a, Lyra, turned back.
b, b, Posterior pillars of the fornix, turned back.
c, c, Anterior pillars of the fornix.
d, Velum interpositum and veins of Galen.
e, Fifth ventricle.
f, f, Corpus striatum.
g, g, Taenia semicircularis.
h, h, Optic thalamus.
k, Choroid plexus.
l, Taenia hippocampi.
m, Hippocampus major in descending cornu.
n, Hippocampus minor.
o, Eminentia collateralis.]

The _optic thalamus_ is a large, somewhat ovoid body situated behind
the corpus striatum, and above the crus cerebri. Its upper surface is
partly seen in the floor of the body of the lateral ventricle, but is
for the most part covered by the fornix and velum interpositum. Its
postero-inferior surface forms the roof of the descending cornu of
the ventricle, whilst its inner surface forms the side wall of the
third ventricle. At its outer and posterior part are two slight
elevations, in close relation to the optic tract, and named
respectively corpus geniculatum internum and externum.

The posterior knob-like extremity of the thalamus is called the
_pulvinar_; this, as well as the two corpora geniculata and the
superior corpus quadrigeminum, is connected with the optic tract.

The _third ventricle_ (see fig. 6) is a cavity situated in the mesial
plane between the two optic thalami. Its roof is formed by the velum
interpositum and body of the fornix; its floor by the posterior
perforated space, corpora albicantia, tuber cinereum, infundibulum,
and optic commissure; its anterior boundary by the anterior pillars of
the fornix, anterior commissure and lamina cinerea; its posterior
boundary by the corpora quadrigemina and posterior commissure. The
cavity of this ventricle is of small size in the living head, for the
inner surfaces of the two thalami are connected together by
intermediate grey matter, named the _middle_ or _soft commissure_.
Immediately in front of the corpora quadrigemina, the white fibres of
the _posterior commissure_ pass across between the two optic thalami.
If the anterior pillars of the fornix be separated from each other,
the white fibres of the _anterior commissure_ may be seen lying in
front of them.

FIG. 14.--Horizontal Section through the Right Cerebral Hemisphere at
the Level of the Widest Part of the Lenticular Nucleus.]

The _pineal body_ is a reddish cone-shaped body situated upon the
anterior pair of the corpora quadrigemina (see figs. 3 and 6). From
its broad anterior end two white bands, the _peduncles_ of the _pineal
body_, pass forward, one on the inner side of each optic thalamus.
Each peduncle joins, along with the taenia semicircularis, the
anterior pillar of the fornix of its own side. In its structure this
body consists of tubular gland tissue containing gritty calcareous
particles, constituting the _brain sand_. Its morphology will be
referred to later.

A general idea of the internal structure of the brain is best obtained
by studying a horizontal section made just below the level of the
Sylvian point and just above the great transverse fissure (see fig.
14). Such a section will cut the corpus callosum anteriorly at the
genu and posteriorly at the splenium, but the body is above the plane
of section. Behind the genu the fifth ventricle is cut, and behind
that the two pillars of the fornix which here form the anterior
boundary of the third ventricle. At the posterior end of this is the
pineal body, which the section has just escaped. To the outer side of
the fornix is seen the foramen of Munro, leading into the front of the
body and anterior horn of the lateral ventricle. It will be seen that
the lateral boundary of this horn is the cut caudate nucleus of the
corpus striatum, while the lateral boundary of the third ventricle is
the cut optic thalamus, both of which bodies have been already
described, but external to these is a third triangular grey mass, with
its apex directed inward, which cannot be seen except in a section.
This is the lenticular nucleus of the corpus striatum, the inner or
apical half of which is of a light colour and is called the _globus
pallidus_, while the basal half is reader and is known as the
_putamen._ External to the putamen is a long narrow strip of grey
matter called the _claustrum_, which is sometimes regarded as a third
nucleus of the corpus striatum. These masses of grey matter, taken
together, are the basal nuclei of the brain. Internal to the
lenticular nucleus, and between it and the caudate nucleus in front
and the thalamus behind, is the _internal capsule_, through which run
most of the fibres connecting the cerebral cortex with the crus
cerebri. The capsule adapts itself to the contour of the lenticular
nucleus and has an anterior limb, a bend or genu, and a posterior
limb. Just behind the genu of the internal capsule is a very important
region, for here the great motor tract from the Rolandic region of the
cortex passes on its way to the crusta and spinal cord. Besides this
there are fibres passing from the cortex to the deep origins of the
facial and hypo-glossal nerves. Behind the motor tracts are the
sensory, including the fillet, the superior cerebellar peduncle and
the inferior quadrigeminal tract, while quite at the back of the
capsule are found the auditory and optic radiations linking up the
higher (cortical) and lower auditory and visual centres. Between the
putamen and the claustrum is the _external capsule_, which is smaller
and of less importance than the internal, while on the lateral side of
the claustrum is the white and then the grey matter of the central
lobe. As the fibres of the internal capsule run up toward the cortex
they decussate with the transverse fibres of the corpus callosum and
spread out to form the _corona radiata._ It has only been possible to
deal with a few of the more important bundles of fibres here, but it
should be mentioned that much of the white matter of the brain is
formed of association fibres which link up different cortical areas,
and which become medullated and functional after birth.

_Weight of the Brain._

This has been the subject of a great deal of research, but the results
are not altogether conclusive; it seems, however, that, although the
male brain is 4 to 5 oz. heavier than that of the female, its relative
weight to that of the body is about the same in the two sexes. An
average male brain weighs about 48 oz. and a female 43-1/2 oz. The
greatest absolute weight is found between twenty-five and thirty-five
years of age in the male and a little later in the female. At birth
the brain weighs comparatively much more than it does later on, its
proportion to the body weight being about 1 to 6. At the tenth year it
is about 1 to 14, at the twentieth 1 to 30, and after that about 1 to
36.5. In old age there is a further slight decrease in proportion. In
many men of great intellectual eminence the brain weight has been
large--Cuvier's brain weighed 64-1/2 oz., Goodsir's 57-1/2, for
instance--but the exceptions are numerous. Brains over 60 oz. in
weight are frequently found in quite undistinguished people, and even
in idiots 60 oz. has been recorded. On the other hand, microcephalic
idiots may have a brain as low as 10 or even 8-1/2 oz., but it is
doubtful whether normal intelligence is possible with a brain weighing
less than 32 oz. The taller the individual the greater is his brain
weight, but short people have proportionally heavier brains than tall.
The weight of the cerebellum is usually one-eighth of that of the
entire brain. Attempts have been made to estimate the surface area of
the grey matter by dissecting it off and measuring it, and also by
covering it with gold leaf and measuring that. The results, however,
have not been conclusive.

Further details of the brain, abundantly illustrated, will be found in
the later editions of any of the standard text-books on anatomy,
references to which will be found in the article on ANATOMY: _Modern
Human. Das Menschenhirn_, by G. Retzius (Stockholm, 1896), and
numerous recent memoirs by G. Elliot Smith and D.J. Cunningham in the
_Journ. Anat. and Phys._ and _Anatomisch Anzeig._, may be consulted.

_Histology of Cerebral Cortex._

The cerebral cortex (see fig. 15) consists of a continuous sheet of
grey matter completely enveloping the white matter of the hemispheres.
It varies in thickness in different parts, and becomes thinner in old
age, but all parts show a somewhat similar microscopic structure.
Thus, in vertical section, the following layers may be made out:--

1. _The Molecular Layer (Stratum zonale)._--This is made up of a large
number of fine nerve branchings both medullated and non-medullated.
The whole forms a close network, the fibres of which run chiefly a
tangential course. The cells of this layer are the so-called _cells of
Cajal_. They possess an irregular body, giving off 4 or 5 dendrites,
which terminate within the molecular layer and a long nerve fibre
process or neuraxon which runs parallel to the surface of the
convolution.

2. _The Layer of small Pyramidal Cells._--The typical cells of this
layer are pyramid-shaped, the apices of the pyramids being directed
towards the surface. The apex terminates in a dendron which reaches
into the molecular layer, giving off several collateral horizontal
branches in its course. The final branches in the molecular layer take
a direction parallel to the surface. Smaller dendrites arise from the
lateral and basal surfaces of these cells, but do not extend far from
the body of the cell. The neuraxon always arises from the base of the
cell and passes towards the central white matter, thus forming one of
the nerve-fibres of that substance. In its path it gives off a number
of collaterals at right angles, which are distributed to the adjacent
grey matter.

Fig. 15.--Diagram to illustrate Minute Structure of the Cerebral
Cortex.

A. Neuroglia cells.
B. " "
C. Cell with short axon (N) which breaks up in a free arborization.
D. Spindle-shaped cell in stratum zonale.
E. Small pyramidal cell.
F. Large pyramidal cell.
G. Cell of Martinotti.
H. Polymorphic cell.
K. Corticipetal fibres.]

3. _The Layer of large Pyramidal Cells._--This is characterized by the
presence of numbers of cells of the same type as those of the
preceding layer, but of larger size. The nerve-fibre process becomes a
medullated fibre of the white matter.

4. _The Layer of Polymorphous Cells._--The cells of this layer are
irregular in outline, and give off several dendrites branching into
the surrounding grey matter. The neuraxon gives off a number of
collaterals, and then becomes a nerve-fibre of the central white
matter.

Scattered through these three layers there are also a number of cells
(_cells of Golgi_) whose neuraxon divides at once, the divisions
terminating within the immediate vicinity of the cell-body. Some cells
are also found in which the neuraxon, instead of running into the
white matter of the brain, passes toward the surface; these are called
_cells of Martinotti_.

The medullated nerve-fibres of the white matter when traced into the
cortex are seen to enter in bundles set vertically to the surface.
These bundles taper and are resolved into isolated fibres in the upper
parts of the pyramidal layers. The fibres constituting the bundles
form two sets. (a) The centrifugal fibres consist as above described
of the fibre processes of the pyramidal and polymorphous cells. (b)
The centripetal fibres ascend through the cortex to terminate within
the molecular layer by horizontally running branches. As they pass
through they give off a number of collaterals. The position of the
cells from which these fibres arise is not known. In addition to the
radially arranged bundles of fibres, networks are formed by the
interlacement with them of large numbers of fine medullated fibres
running tangentially to the surface. These are derived chiefly from
the collaterals of the pyramidal cells and of the centripetal fibres.
They form two specially marked bundles, one within the layer of the
polymorphous cells known as the _inner band of Baillarger_, and
another in the layer of large pyramidal cells called the _outer band
of Baillarger_. This latter is very thick in the calcarine region, and
forms the _white stria of Gennin_, while the inner band is best seen
in the precentral gyrus. As both these strands cross the already
mentioned radial bundles at right angles, they are regarded as
specialized parts of an _interradial reticulum_ of fibres, but, nearer
the surface than the radial bundles penetrate, tangential fibres are
found, and here they are called the _supraradial reticulum_. In
certain parts of the brain the fibres of this reticulum are more
closely set, and form the _band of Bechterew_ in the superficial part
of the small pyramidal cell zone.

Fig. 16.--Brain of _Petromyzon marinus_ (dorsal view). A, Brain; B,
choroid plexus removed.]

For further information on the structure of the cerebral cortex, see
A.W. Campbell, _Proc. R. Soc._ vols. lxxii. and lxxiv.

_Comparative Anatomy._

A useful introduction to the study of the vertebrate brain is that of
the Amphioxus, one of the lowest of the Chordata or animals having a
notochord. Here the brain is a very slightly modified part of the
dorsal tubular nerve-cord, and, on the surface, shows no distinction
from the rest of that cord. When a section is made the central canal
is seen to be enlarged into a cavity, the neurocoele, which, in the
young animal, communicates by an opening, the neuropore, with the
bottom of the olfactory pit, and so with the exterior. More ventrally
another slight diverticulum probably represents the infundibulum. The
only trace of an eye is a patch of pigment at the anterior end of the
brain, and there are no signs of any auditory apparatus. There are
only two pairs of cerebral nerves, both of which are sensory (Willey,
_Amphioxus_, 1894). In the Cyclostomata, of which the lamprey
(Petromyzon) is an example, the minute brain is much more complex,
though it is still only a very slight enlargement of the anterior end
of the cord. The single cavity seen in Amphioxus is here subdivided
into three: an anterior or prosencephalon, a middle or mesencephalon,
and a hinder or rhombencephalon. The rhombencephalon has a very slight
transverse thickening in the fore-part of its roof, this is the
rudimentary cerebellum (_Cer._); the rest of this part of the brain is
taken up by the large medulla, the cavity of which is the _fossa
rhomboidalis_ or fourth ventricle. This fossa is roofed over by the
epithelium lining the cavity of the ventricle, by pia mater and
blood-vessels constituting a choroid plexus (fig. 16, B). The fourth
ventricle communicates with the parts in front by means of a passage
known as the aqueduct of Sylvius.

The mesencephalon or mid-brain, when looked at from the dorsal
surface, shows a pair of large hollow swellings, the optic lobes or
_corpora bigemina_. Their cavities open out from the aqueduct of
Sylvius, and from the nervous tissue in their walls the optic nerves
derive their fibres. From the front of the prosencephalon or anterior
vesicle the olfactory nerves come off, and at the base of each of
these are two hollow swellings; the larger and more anterior is the
olfactory bulb, the smaller and more posterior the cerebral
hemisphere. Both these swellings must be regarded as lateral
outgrowths from the blind front end of the original single vesicle of
the brain as seen in Amphioxus, and from the anterior subdivision or
prosencephalon in the lamprey. The anterior vesicle, however, is now
again subdivided, and that part from which the cerebral hemispheres
bud out, and the hemispheres themselves, is called the telencephalon,
while the posterior part of the original prosencephalon is known as
the thalamencephalon, or more rarely the diencephalon. On the dorsal
surface of the thalamencephalon are two nervous masses called the
ganglia habenulae; the right is much larger than the left, and from it
a stalk runs forward and upward to end in the vestigial pineal body
(or epiphysis), which contains rudiments of a pigmented retina and of
a lens, and which is usually regarded as the remains of one of a pair
of median eyes, though it has been suggested that it may be an organ
for the appreciation of temperature. From the small left ganglion
habenulae a still more rudimentary pineal stalk projects, and there
are signs of a third outgrowth (paraphysis) in front of these. On the
floor of the thalamencephalon the blind pouch-like infundibulum is in
contact with the pituitary body, an outgrowth from the combined
pituitary and olfactory pouch, which in the adult opens on to the top
of the head just in front of the pineal area. The anterior closed end
of the nerve-tube, in front of the foramina of Munro or openings from
which the hemispheres have grown out, is known as the _lamina
terminalis_, and in this is seen a little white commissure, connecting
the hemispheres of opposite sides and belonging entirely to the
telencephalon, known as the anterior commissure. The roof of the
telencephalon is mainly epithelial, and contains no traces of cortical
structure. In the posterior part of the roof of the thalamencephalon
is the small posterior commissure (Ahlborn, _Zeits. wiss. Zool._ Bd.
xxxix., 1883, p. 191). In the Elasmobranch Fish, such as the sharks
and rays, the cerebellum (_Cer._ fig. 17) is very large and contains
the layers found in all the higher vertebrates. In the mesencephalon
fibres corresponding with those of the fillet of higher vertebrates
can be seen, and there is a nucleus in the hinder part of the _corpora
bigemina_ foreshadowing the separation into corpora quadrigemina.
There is only one pineal stalk in the roof of the thalamencephalon,
and the ganglia habenulae--very constant structures in the vertebrate
brain--are not so marked as in Petromyzon, but are, as usual,
connected with the olfactory parts of the cerebrum, with the surface
of the optic lobes (_tectum opticum_), and with the _corpus
interpedunculare_ (Meynert's bundle). They are united across the
middle line by a small _superior_ or _habenular commissure_. In the
floor of the thalamencephalon are two masses of ganglionic tissue, the
optic thalami. The infundibulum dilates into two rounded bodies, the
_lobi inferiores_, while the pituitary body or _hypophysis cerebri_
has two lateral diverticula known as _sacci vasculosi_. Ganglia
geniculata are found for the first time in connexion with the optic
tracts in the lower part of the thalamus. The olfactory lobes (fig.
17, _Olf. Bulb_) are very large and often separated by long stalks
from the cerebral hemispheres, which are comparatively much larger
than those of the Cyclostomata; their roof or pallium is nervous, but
devoid of cortical structure, while in the floor in some species large
anterior basal ganglia or _corpora striata_ are found
(Miklucho-Maclay, _Beitrage z. vergl. Neurol._, 1870; Edinger, _Arch.
mikr. Anat._ Bd. lviii., 1901, p. 661, "Cerebellum"). The Teleostean
Fish are chiefly remarkable for the great development of the optic
lobes and suppression of the olfactory apparatus. The pallium is
non-nervous, and the optic tracts merely cross one another instead of
forming a commissure. A process of the cerebellum called _valvula
cerebelli_ projects into the cavity of each optic lobe (Rabl.
Ruckhard, _Arch. Anat. u. Phys_., 1898, p. 345 [Pallium]; Haller,
_Morph. Jahrb._ Bd. xxvi., 1898, p. 632 [Histology and Bibliography]).
The brain of the Dipnoi, or mud fish, shows no very important
developments, except that the anterior pineal organ or paraphysis is
large (Saunders, _Ann. and Mag. Nat. Hist._ ser. 6, vol. iii., 1889,
p. 157; Burkhardt, _Centralnervensystem v. Protopterus_, Berlin,
1892).

FIG. 17.--Section of the Brain of Porbeagle Shark (_Lamna_).]

In the Amphibia the brain is of a low type, the most marked advances
on that of the fish being that the anterior commissure is divided into
a dorsal and ventral part, of which the ventral is the true anterior
commissure of higher vertebrates, while the dorsal is a hippocampal
commissure and coincides in its appearance with the presence of a
small mass of cells in the outer layer of the median wall of the
pallium, which is probably the first indication of a hippocampal
cortex or cortex of any kind (Osborn, _Journ. Morph._ vol. ii., 1889,
p. 51).

Fig. 18.--Section of Brain of Turtle (_Chelone_).]