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Pons Varolli.

The grey matter of the medulla oblongata, which contains numerous multipolar nerve cells, is in part continuous with the grey matter of the spinal cord, and in part consists of independent masses. As the grey matter of the cord enters the medulla it loses its crescentic arrangement. The posterior cornua are thrown outwards towards the surface, lose their pointed form, and dilate into rounded masses named the grey tubercles of Rolando, whilst portions are prolonged into both the posterior pyramid and the restiform body. The grey matter of the anterior cornua and of the intermediolateral tracts loses its continuity, and becomes subdivided into numerous small masses, owing to being traversed by bundles of nerve fibres, which give rise to a network termed formatio reticularis, in the meshes of which the groups of nerve cells are contained. In the lower part of the medulla a central canal continuous with that of the cord exists, but when the restiform bodies and posterior pyramids on the opposite sides of the medulla diverge from each other, the central canal loses its posterior boundary, and dilates into the cavity of the 4th ventricle. The grey matter in the interior of the medulla appears, therefore, on the floor of the ventricle; that which corresponds to the anterior cornua being situated immediately on each side of the median furrow, whilst that which is continuous with the grey tubercles of Rolando and the posterior cornua is some distance external to it. This grey matter forms collections of nerve cells, which are the centres of origin of several important encephalic nerves.

Of the independent masses of grey matter of the medulla, that which forms the corpus dentatum within the olivary body is the most important, and constitutes the nucleus of the inferior olive. It is folded on itself in a zig-zag or denticulated manner, and forms a sort of capsule open on the inner aspect, through which openings a bundle of nerve fibres from the interior of the capsule proceeds. These fibres aid in the formation of the olivary fasciculus, and as Deiters and Meynert have pointed out, in part arch across the mesial plane and join the restiform body on the opposite side, whilst some apparently join the posterior pyramid. The nerve cells of the olive are multipolar and flask-shaped, and in all probability give origin to the nerve fibres proceeding from the interior of the capsule. Separated from the inner part of the olive by a layer of reticular substance is a smaller grey mass, called by Stilling nucleus olivaris accessorius. Crossing the anterior surface of the medulla oblongata, immediately below the pons, in the majority of mammals is a transverse arrangement of fibres forming the trapezium, which contains a grey nucleus, named by Van der Kolk the superior olive. In the human brain the trapezium is concealed by the lower transverse fibres of the pons, but when sections are made through it, as L. Clarke pointed out, the grey matter of the superior olive can be seen. Meynert states that its nerve cells give origin to some fibres, which run straight backwards to the restiform body of the same side, and to others which pass across the mesial plane to the opposite corpus restiforme.

The PONS VAROLII or BRIDGE (Pl. XVIII. figs. 1, 2, 3, N) is cuboidal in form: its anterior surface rests upon the dorsum sellæ of the sphenoid, and is marked by a median longitudinal groove; its inferior surface receives the pyramidal and olivary tracts of the medulla oblongata; at its superior surface are the two crura cerebri; each lateral surface is in relation to a hemisphere of the cerebellum, and a peduncle passes from the pons into the interior of each hemisphere; the posterior surface forms in part the upper portion of the floor of the 4th ventricle, and in part is in contact with the corpora quadrigemina.

The pons consists of white and grey matter: the nerve fibres of the white matter pass through the substance of the pons, either in a transverse or a longitudinal direction.

The transverse fibres go from one hemisphere of the
cerebellum to that of the opposite side; some are situated
on the anterior surface of the pons, and form its superficial
transverse fibres, whilst others pass through its substance
and form the deep transverse fibres. The transverse fibres
of the pons constitute, therefore, the commissural or
connecting arrangement by which the two hemispheres of
the cerebellum become anatomically continuous with each
other. The longitudinal fibres of the pons ascend or pass
vertically upwards from the medulla oblongata, and consist
of the fibres of the anterior pyramids, olivary fasciculi,
fasciculi teretes, and posterior pyramids. They leave the
pons by emerging from its upper surface as fibres of the
two crura cerebri. The pons possesses a median raphe
continuous with that of the medulla oblongata, and formed
like it by a decussation of fibres in the mesial plane.
The grey matter of the pons is scattered irregularly
through its substance, and appears on its posterior surface;
but not on the anterior surface, which is composed exclu-
sively of the superficial transverse fibres. It is traversed
both by the longitudinal and deep transverse fibres, which
form a well-defined formatio reticularis. To a portion of
grey matter, containing nerve cells charged with dark
pigment, the name of locus cæruleus is applied. The locus
lies on the floor of the 4th ventricle, close to the entrance
to the aqueduct of Sylvius, and serves as the origin of the
sensory root of the 5th, and perhaps of the posterior root of
the 4th cranial nerve. The nerve cells of the pons are multi-
polar and stellate. The pons acts as a conductor of
impressions through its nerve fibres, and as a centre of
origin of nerve fibres from nerve cells. Meynert states that
some of the fibres of the crura cerebri end in the nerve
cells of the pons, which cells again give origin to fibres
that pass outwards to the cerebellum.

The CEREBELLUM, LITTLE BRAIN, or AFTER BRAIN (Pl. CereXVIII. fig. 2, c), occupies the inferior pair of occipital fosse, bellum. and, along with the pons and medulla oblongata, 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 vermiform process. It is connected below with the medulla oblongata by the two restiform bodies which form its inferior peduncles, and above to 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 inferior vermiform 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 cerebral 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 posterior 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 lamellæ by numerous fissures. The cerebellum consists both of grey and white matter. The grey matter forms the exterior or cortex of the lamellæ, 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 lamellæ give to the section an arborescent appearance, known by the fanciful name of arbor vitæ (Pl. XVIII. fig. 3, c). 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 zig-zag 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. Stilling has described, in connection with the anterior end of the inferior vermiform process, which projects forwards into the valve of Vieussens, and aids in the formation of the roof of the 4th ventricle, two grey masses, named roof nuclei. They possess flask-shaped nerve cells like those of the corpus dentatum. 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 upwards 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 both with the grey matter of the corpus dentatum and of the roof nuclei. The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, reach the grey cortical matter, more especially

axial cylinder of a medullated nerve fibre; for the nerve
fibres of the white core enter this layer, divide into minute
fibres, and ramify amidst the granules. From the oppo-
site aspect of each cell two peripheral processes arise,
and ramify in an antler-like manner in the external grey
layer. Obersteiner and Hadlich maintain that the finer
branches of these processes curve back towards the rust
coloured layer, where, according to Boll, they form a net-
work of extreme minuteness, from which it is believed
that nerve fibres may arise. The substratum of the grey
layer, in which the branched processes of the cells of
Purkinje lie, consists of a very delicate neuroglia, in which
scattered corpuscles are imbedded; but, in the outer part
of this layer, delicate supporting connective tissue-like
fibres are also met with.

The Fourth Ventricle is the dilated upper end of the cen- Fourth tral canal of the medulla oblongata. Its shape is like an Ventricle. heraldic lozenge. Its floor is formed by the grey matter of

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gland; 2, the nates, and 3, the testes of the corpora quadrigemina; 4.4
middle peduncles, 5, 5, superior peduncles, 9, 9, inferior peduncles of the
cerebellum; 6, 6, valve of Vieussens divided; 7, 7, fasciculi teretes; 8, 8, roots
of the auditory nerves; 9', corpus dentatum; 10, 10, posterior pyramids; 11,
calamus scriptorius.

on the inferior surface of the cerebellum, though they also FIG. 68.-Floor of the fourth ventricle and adjacent structures. 1, pine
form connections 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. But, in addition to these
peduncular fibres, which connect the cerebellum to other
subdivisions of the encephalon, its white matter contains
fibres proper to the cerebellum itself. The fibræ proprice
have been especially described by Stilling; some, which he
has termed the median fasciculi, lie near the mesial plane,
and connect the grey matter on the tentorial aspect of the
middle lobe with that of the inferior vermiform process,
whilst others cross directly the mesial plane to unite opposite
and symmetrical regions of the hemispheres. Further, the
auditory nerve was said by Foville to derive some of its
fibres of origin from the cerebellum; the connection of this
nerve with the cerebellum has been strongly insisted on by
Meynert, and this anatomist has also ascribed a cerebellar
origin to a portion of the sensory root of the 5th cranial

nerve.

The grey matter of the cortex is divided into two welldefined layers, an external grey, and an inner rust coloured layer of about equal thickness. The rust coloured layer is distinguished by containing multitudes of so called "granules," the well-defined nucleus in which, as described by Strachan, is invested by a small quantity of branched protoplasm. These "granules" are, therefore, minute stellate cells. Where the rust coloured layer joins the grey layer the characteristic nerve cells of the cerebellum, named the corpuscles of Purkinje, are situated. A slender central process arising from each cell enters the rust coloured layer, and, as the observations of Hadlich and Koschennikoff show, becomes continuous with the

the posterior surfaces of the medulla oblongata and pons;
its roof partly by the inferior vermiform process of the cere-
bellum, the nodule of which projects into its cavity, and
partly by a thin layer, called valve of Vieussens, or anterior
medullary velum; its lower lateral boundaries, by the diver-
gent restiform bodies and posterior pyramids; its upper
lateral boundaries, by the superior peduncles of the cere-
bellum; the reflection of the arachnoid membrane from the
back of the medulla to the inferior vermiform process closes
it in below, but allows of a communication between its cavity
and the sub-arachnoid space; above, it communicates with
the aqueduct of Sylvius, which is tunnelled through the sub.
stance 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, striæ acous-
tice, and deposits of grey matter described in connection
with the medulla oblongata. Its endothelial lining is con-
tinuous with that of the central canal.

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 convo-
lutions, the corpora striata, and the optic thalami, developed
in the anterior cerebral vesicle, but also the corpora quadri-
gemina and crura cerebri developed in the middle cerebral
vesicle. The cerebrum is ovoid in shape, and presents
superiorly, anteriorly, and posteriorly a deep median longitu

volution, and passing forwards to the anterior end of the cerebrum, are three convolutions, arranged in parallel tiers from above downwards, and named superior, middle, and inferior frontal convolutions, which are also prolonged on to the orbital face of the frontal lobe. The Parietal Lobe is also complex; its most anterior convolution, named ascending parietal, ascends parallel to and immediately behind the fissure of Rolando. Springing from the upper end of the back of this convolution is the postero-parietal convolution, 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 convolution is the supra-marginal convolution, which forms the upper boundary of the hinder part of the Sylvian

dinal 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 longi-
tudinal 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 forwards and upwards to enter the optic
thalami in their respective hemispheres. 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,
called pons Tarini, which, from being perforated by seve-
ral small arteries, is often called locus perforatus posticus; Oc
b, two white mammillæ, the corpora albicantia; c, a grey
nodule, the tuber cinereum, from which, d, the infundi-
bulum projects to join the pituitary body. Immediately in
front of the optic commissure is a grey layer,
the lamina
cinerea or lamina terminalis of the 3d 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.

The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as the convolutions or gyri of the cerebrum. These convolutions are separated from each other by fissures or sulci, some of which are considered to subdivide, the hemisphere into lobes, whilst others separate the convolutions in each lobe from each other. In each hemisphere of the human brain five lobes are recognised: the temporo-sphenoidal, frontal, parietal, occipital, and the central lobe or insula. Passing obliquely on the outer face of the hemisphere from before, upwards and backwards, is the wellmarked Sylvian fissure, 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. The frontal lobe is separated from the parietal by the fissure of Rolando, which extends on the outer face of the hemisphere from the longitudinal fissure obliquely downwards and forwards towards the Sylvian fissure. About two inches 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.

The Temporo-Sphenoidal Lobe presents on the outer surface of the hemisphere three convolutions, arranged in parallel tiers from above downwards, and named superior, middle, and inferior temporo-sphenoidal convolutions. The fissure which separates the superior and middle of these convolutions is called the parallel fissure. The Occipital Lobe also consists from above downwards of three parallel convolutions, 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, which ascends obliquely backwards and upwards from the Sylvian to the longitudinal fissure. Springing from the front of this con

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FIGS. 69 and 70.-Profile and vertex views of cerebrum. Fr, the frontal lobe; Par, parietal; Oc, occipital; Ts, temporo-sphenoidal lobe; SS, Sylvian fissure; RR, fissure of Rolando; PO, parieto-occipital fissure; IP, intra-parietal fissure; PP, Parallel fissure; SF and IF, supero- and infero-frontal fissures; 1, 1, 1, inferior, 2, 2, 2, middle, and 3, 3, 3, superior frontal convolutions; 4, 4, ascending frontal convolution; 5, 5, 5, ascending parietal, 5', postero-parietal, and 6, 6, angular convolutions; A, supra-marginal, or convolution of the parietal eminence; 7, 7, superior, 8, 8, 8, middle, and 9, 9, 9, inferior temporosphenoidal convolutions; 10, superior, 11, middle, and 12, inferior occipital convolutions; a, B, y, d, four annectent convolutions.

fissure; as this gyrus occupies the hollow in the parietal bone, which corresponds to the eminence, it may appropriately be named the convolution of the parietal eminence. Continuous with the convolution of the parietal eminence is the angular convolution, which bends round the posterior extremity of the Sylvian fissure. Lying in the parietal lobe is the intra-parietal fissure, which separates the convolution of the parietal eminence from the postero-parietal conI. — 110

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FIG. 71.-Side view of the Brain in the skull.1 depth and extent of this fissure vary in different brains in proportion to the size of these bridging convolutions. The superior annectent gyrus passes between the postero-parietal and the superior occipital convolutions, whilst the second annectent gyrus connects the middle occipital with the angular gyrus.. Two annectent gyri also pass from the inferior occipital convolution to the lower convolutions of the temporosphenoidal lobe. These lobes of the cerebrum, though

named after the bones which form the vault of the skull, are not exactly co-terminous with them. The frontal lobe not only lies under cover of the frontal bone, but extends backwards under the anterior part of the parietal; for the fissure of Rolando, which forms its posterior boundary, lies from 1 to 2 inches behind the coronal suture. The occipital lobe is not limited to the upper tabular part of the occipital bone, but extends forwards under cover of the posterior part of the parietal, for the parieto-occipital fissure lies about inch in front of the apex of the lambdoidal fissure. The temporo-sphenoidal lobe not only lies under the squamous-temporal and great wing of the sphenoid, but passes upwards under cover of the lower part of the parietal, for the Sylvian fissure passes from below obliquely upwards and backwards across the line of the squamous suture near its middle. The area covered by the parietal bone so far, 1 The above view of the brain in situ shows the relations of the surface convolutions to the regions of the skull. R, fissure of Rolando, which separates the frontal from the parietal lobe. PO, parieto-occipital fissure between the parietal and occipital lobes. SS, fissure of Sylvius, which separates the temporo-sphenoidal from the frontal and parietal lobes. SF, MF, IF, the supero-, mid-, and infero-frontal subdivisions of the frontal area of the skull; the letters are placed on the superior, middle, and inferior frontal convolutions; the inferior frontal region is separated from the middle frontal by the frontal part of the curved line of the temporal ridge; the mid- from the supero-frontal by an antero-posterior line through the frontal eminence. SAP, the superoantero-parietal area of the skull; S is placed on the ascending parietal convolution, AP on the ascending frontal convolution. IAP, the inferoantero-parietal area of the skull; I is placed on the ascending parietal, AP on the ascending frontal convolution. SPP, the supero-postero-parietal area of the skull; the letters are placed on the angular convolution. IPP, the infero-postero-parietal area of the skull; the letters are placed on the mid-temporo-sphenoidal convolution; the temporal ridge separates the supero- and infero-parietal regions from each other; a vertical line drawn through the parietal eminence separates the antero- and posteroparietal regions. X, the convolution of the parietal eminence, or supramarginal gyrus. O, the occipital area of the skull; the letter is placed on the mid-occipital convolution. Sq, the squamoso-temporal region of the skull; the letters are placed on the mid-temporo-sphenoidal convolution. AS, the ali-sphenoid region of the skull; the letters are placed on the tip of the supero-temporo-sphenoidal convolution. The black lines mark the boundaries of different cranial regions.

| then, from being co-terminous with the parietal lobe of the cerebrum, is trenched on anteriorly by the frontal, posteriorly by the occipital, and inferiorly by the temporosphenoidal lobe. The convolutions of the parietal lobe itself are grouped around the parietal eminence, and in the interval between it and the sagittal suture. The inner table of the cranial bones is an almost exact mould of the convolutions of these lobes; but this is not so with the exterior of the skull, the configuration of which is modified by the formation of ridges and processes for the attachment of muscles, by variations in the thickness of the diploë, and by the development of the frontal and mastoid airsinuses. Hence the outer surface of the skull does not correspond in shape to the outside of the brain.

The Central Lobe of the hemisphere, more usually called the insula or island of Reil, does not come to the surface of the hemisphere,

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but lies deeply within the Sylvian fissure, the convolutions forming the margin of which conceal it. It consists of four or five short convolutions, which radiate from the locus perforatus anticus, situ

ated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus,

which insulates it from the adjacent convolutions. It lies opposite the upper part of the ali articulates with the sphenoid, where it parietal and squamous-temporal.

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TR

FIG. 72.-Orbital surface of the left frontal lobe

Convolutions also exist on the inner surface of the hemisphere, and on the under surface which rests on the tentorium, but these have no relation to the bones of the cranial vault. They may be studied in connection with the corpus callosum or great transverse commissure, which connects the two hemispheres, and with certain fissures situated on these surfaces of the hemisphere. The small convolutions which lie behind the internal part of the parieto-occipital fissure form the inner convolutions of the occipital lobe, or the occipital lobule (Fig. 73). Those which lie immediately in front of the same fissure belong to the inner face of the parietal lobe, and form the quadrilateral lobule. It is customary, however, to name the convolution which extends forwards from that fissure along the margin of the longitudinal fissure to the anterior end of the hemisphere, and which then turns back to the locus perforatus anticus as the marginal convolution. This is separated by a fissure called calloso-marginal, from the callosal convolution or gyrus fornicatus, which, commencing at the locus perforatus anticus, turns round the anterior end of the corpus callosum, extends parallel to its upper surface, and then turns round its posterior end. It is separated from the corpus callosum by the callosal fissure, at the bottom of which the grey matter of the gyrus fornicatus termintes in a well-defined edge.

and the island of Reil; the tip of the temporosphenoidal lobe has been removed to display the tudinal fissure; 0, olfactory fissure, over which the olfactory peduncle and lobe are situated: orbital surface; 1, 1, 1, 1, under surface of inferoing frontal, and 5, of ascending parietal convolutions; C, central lobe or insula.

latter. 17, convolution of the margin of the longi

TR, tri-radiate fissure; 1" 1", convolutions on the frontal convolution; 4, under surface of ascend

The callosal convolution encloses the corpus callosum | the surface of the corpus callosum a few fibres, the stric within the concavity of its arch, and from its direction is longitudinales, run in the antero-posterior or longitudinal Par

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FIG. 73.-Convolutions of the inner and tentorial surfaces of the left hemisphere. i, i, i, calloso-marginal fissure; 1, 1, calcarine fissure; m, m, hippocampal fissure; n, n, collateral fissure; PO, parieto-occipital fissure; 17, 17, marginal convolution; 18, 18, gyrus fornicatus; 18', quadrilateral lobule; 19, hippocampal gyrus; 19', its recurved end; 25, occipital lobule; 9, 9, inferior temporo-sphenoidal convolution.

appropriately called fornicatus (arch-shaped). The posterior end of the callosal convolution curves downwards and then forwards, under the name of gyrus hippocampi, to the tip of the inner surface of the temporo-sphenoidal lobe. This gyrus is separated anteriorly by a narrow curved fissure called hippocampal fissure, from a white band, the tænia hippocampi, which band possesses a free curved border, round which the pia mater and choroidal artery enter the lateral ventricle through the great transverse fissure of the cerebrum. The hippocampal fissure is continuous round the posterior end of the corpus callosum with the callosal fissure, and at the bottom of the hippocampal fissure the grey matter of the gyrus hippocampi terminates in a well-defined dentated border (fascia dentata). The hippocampal fissure on this surface of the hemisphere marks the position of an eminence in the descending cornu of the ventricle called hippocampus major. The gyrus hippocampi is separated posteriorly from the adjacent temporo-sphenoidal convolution by a fissure, named collateral, which marks the position on this surface of the hemisphere of the collateral eminence in the interior of the ventricle. From the lower end of the parieto-occipital fissure an offshoot, called the calcarine fissure, passes almost horizontally backwards in the occipital lobe, which fissure marks on this surface of the hemisphere the eminence named calcar avis, or hippocampus minor, in the posterior cornu of the ventricle.

If a horizontal slice be removed from the upper part of each hemisphere, the peripheral grey matter of the convolutions will be seen to follow their various windings, whilst the core of each convolution 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. The corpus callosum does not equal the hemispheres in length, but approaches nearer to their anterior than their posterior ends (Pl. XVIII. fig. 3, B.) It terminates behind in a free rounded end, 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 convolutions, and apparently connect the corresponding convolutions in the opposite hemispheres. Hence the corpus callosum is a connecting or commissural structure, which brings the convolutions of the two hemispheres into anatomical and physiological relation with each other. On

FIG. 74.-To show the right ventricle and the left half of the corpus callosum. 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, tænia semicircularis; h, optic thalamus; k, choroid plexus; 1, tænia hippocampi; m, hippocampus major; n, hippocampus minor; o, eminentia collateralis.

direction. 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.

The lateral ventricle is subdivided into a central space or body, and three bent prolongations or cornua; the anterior cornu extends forwards and outwards into the frontal lobe; the posterior cornu curves backwards, outwards, and inwards into the occipital lobe; the descending cornu curves backwards, outwards, downwards, forwards, and inwards, behind and below the optic tha lamus into the temporo-sphenoidal lobe. On the floor of the central space may be seen from before backwards the grey upper surface of the pear-shaped corpus striatum, 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. 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, and between this cornu and the descending cornu is the clevation called 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 tania hippocampi, continuous above with the posterior pillar of the fornix. If the tænia be drawn on 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 dentata). The choroid plexus of the pia mater turns round the gyrus hippocampi, and enters the descending cornu through the great transverse fissure between the tænia hippocampi and optic thalamus. The lateral ventricle is lined by a cylindrical endothelium,

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