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upper end of the hippocampal formation so that it becomes removed far from the lamina terminalis. The fibres of the fimbria which are prolonged forwards under the corpus callosum and septum pellucidum to bridge this great gap form the crus fornicis on each side. As a rule in the human adult brain the crura fornicis of the two hemispheres become crowded together at the median plane so as to obscure the connecting lamella which serves as a matrix for the commissura hippocampi (Fig. 557, C); but the true arrangement can be seen in the brains of foetuses of the sixth, seventh, and eighth months, and is at once revealed in the adult if the corpus callosum is raised up by an accumulation of fluid in the lateral ventricles (hydrocephalus), so as to put a strain upon the septum pellucidum. The mass formed by the crura fornicis and their commissure is called the corpus fornicis.

The fascia dentata appears as a notched band behind and below the fimbria; its upper end passes on to the under surface of the splenium of the 'corpus callosum, where it tapers and ends (fasciola cinerea); but as it dwindles the upper end of the hippocampus emerges upon the surface below and behind it and passes into a thin film of gray matter-indusium griseum-which is prolonged on to the upper surface of the corpus callosum. It proceeds forwards, becoming as a rule still

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Epithelial
roof of third
ventricle

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Olfactory bulb Vestiges of the precallosal. hippocampus

Lamina terminalis

Septum pellucidum

Paraterminal body

MRoof

Commissura
hippocampi

Rostrum

wir Roof

Paraterminal body

FIG. 557.-THREE STAGES IN THE DEVELOPMENT OF THE CORPUS CALLOSUM.

Commissura hippocampi

more attenuated, and after surrounding the anterior end (genu) of the corpus. callosum it passes downwards towards the trigonum olfactorium along the line that separates the corpus paraterminale from the neopallium. The indusium represents the atrophied remains of the anterior part of the hippocampal arc of the foetal brain (Fig. 555), from which the fascia dentata has entirely disappeared. It is accompanied by longitudinal fibres homologous to the fornix system: in other words, the fornix fibres of the atrophied supracallosal hippocampus; they form the striæ longitudinales of the corpus callosum (Fig. 558; Fig. 564, p. 635; Fig. 559, p. 631).

The inferior (or anterior) extremity of the fascia dentata dips into a deep furrow, around which the area piriformis is bent in a hook-like manner (uncus); in this it becomes considerably reduced in diameter and then emerges (at right angles to its previous direction) to form Giacomini's "banderella," which we may call the cauda fascia dentatæ. Behind this the inferior end of the hippocampus comes to the surface, but is turned inside out, hippocampus inversus. Just in front of the upper ending of the cauda fascia dentatæ a little knob of solid gray matter appears upon the surface, surrounded by area piriformis. It is the nucleus amygdala (Fig. 558).

Corpus Callosum.-The corpus callosum is the great transverse commissure which passes between the two cerebral hemispheres. It is placed nearer the anterior than the posterior aspect of the brain, and it unites the medial surfaces of the hemispheres throughout very nearly a half of their antero-posterior

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cinguli. The thin coating of gray matter, with the two striæ, represents the aborted remains of the hippocampus (see p. 627). So thin is this gray coating that the transverse direction pursued by the callosal fibres proper can be easily perceived through it.

The two extremities of the corpus callosum are much thickened, whilst the intermediate part or body is considerably thinner. The massive posterior end, which is full and rounded, lies over the mesencephalon and extends backwards as far as the highest point of the cerebellum. It is called the splenium, and it consists of a superior and inferior part. The latter is bent forwards under the upper part, to the inferior surface of which it is closely applied. The anterior end of the corpus callosum is not quite so massive, and it is folded downwards and backwards on itself. It is termed the genu. The recurved inferior part of the genu is separated from the part of the corpus callosum which lies above, by an interval. It rapidly thins as it passes backwards and receives the name of the rostrum. The fine terminal edge of the rostrum becomes connected by means of a band of neuroglial

tissue with the lamina terminalis on the antero-superior aspect of the anterior commissure (Fig. 558).

The inferior surface of the corpus callosum, on each side of the median plane, is coated with ependyma (Fig. 564, p. 635), and forms the roof of the anterior horn and the central part of the lateral ventricle. In the median plane, however, it is attached to subjacent parts, viz., to the septum pellucidum in front and directly or indirectly (Fig. 564) to the body of the fornix behind (Fig. 558, p. 630).

The transverse fibres of the corpus callosum, as they enter the white medullary centre of the cerebral hemisphere, separate from each other so as to reach most parts of the cerebral cortex. These diverging fibres are termed the radiatio corporis

Corpus callosum

Cingulum

Genu

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Transverse fibres of corpus callosum

Inferior longitudinal bundle

Occipital part of radiation of corpus callosum.

Splenium

Stria longitudinalis medialis

FIG. 559.-THE CORPUS CALLOSUM, exposed from above and the right half dissected,
to show the course taken by its fibres.

The lateral longitudinal stria (which lies near the cingulum) is not shown.

callosi, and they intersect those which form the corona radiata or, in other words, the fibres which extend between the internal capsule and the cerebral cortex (Figs. 570, p. 640, and 576, p. 649). The more anterior of the fibres which compose the genu of the corpus callosum sweep forwards in a series of curves into the anterior frontal region of the hemisphere. A large part of the splenium, forming a solid bundle termed the occipital part of the radiation of the corpus callosum (O.T. forceps major), bends suddenly and abruptly backwards into the occipital lobe (Fig. 559). Fibres from the body and superior part of the splenium, curving round the lateral ventricle, form a very definite stratum, called the tapetum. This is a thin layer in the medullary centre of the hemisphere, which constitutes the immediate roof and lateral wall of the posterior horn and the lateral wall of the posterior part of the inferior horn of the lateral ventricle

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VENTRICULUS LATERALIS.

The cavity in the interior of the cerebral hemisphere is called the lateral ventricle. It is lined throughout by ependyma continuous with the ependymal lining of the third ventricle. In some places the walls of the cavity are in apposition, whilst in other localities spaces of varying capacity, and containing cerebro-spinal fluid, are left between the bounding walls.

The lateral ventricle communicates with the third ventricle of the brain by means of a small foramen, just large enough to admit a crow-quill, which is termed the foramen interventriculare. This aperture is placed in front of the anterior end of the thalamus and behind the column of the fornix.

The highly-irregular shape of the lateral ventricle can be best understood by the

study of a cast of its interior
(Figs. 561 and 545, p. 618).
It is usual to describe it as
being composed of a body and
three horns, viz. an anterior, a
posterior, and an inferior horn.
The cornu anterius is that part
of the cavity which lies in
front of the interventricular
foramen. The body or pars
centralis is the portion of the
ventricle which extends from
the interventricular foramen
to the splenium of the corpus
callosum. At this point the
posterior and inferior horns
diverge from the posterior
part of the body. The cornu
posterius curves backwards and
medially into the occipital
lobe. It is very variable in
its length and capacity: the
chief reason for this variability
is that adhesions between the
walls of this part of the ven-
tricle are of common occur-
rence. The cornu inferius
proceeds with a bold sweep
round the posterior end of the
thalamus, and then tunnels in
a forward and medial direction
through the temporal lobe

Longitudinal

fissure

Corpus callosum
Lateral ventricle

Column of fornix
Chorioid plexus_

Foramen interventriculare

Septum pellucidum

H ming

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The early foetal lateral ventricle is very capacious and presents an arched or semilunar form. It is composed of parts which correspond

to

the anterior horn, the central part and the inferior horn, and there is little or no demarcation between them. The posterior horn is a later production. comes into existence as a diver

It

Caudate nucleus
Internal capsule

Nucleus lentiformis

Claustrum

FIG. 562.-FRONTAL SECTION THROUGH THE CEREBRAL HEMISPHERES so as to cut ticulum or elonthrough the anterior horns of the lateral ventricles, through which the central

part of the ventricles, the columns of the fornix, and the interventricular gated

pouch, grows

which

foramina can be seen.

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Vent. III. Third ventricle.

FIG. 561.-DRAWING TAKEN FROM A CAST OF THE VENTRICULAR (After Retzius.) SYSTEM OF THE BRAIN, as seen from above.

CORNU INFERIUS

Vent. IV. Fourth ventricle. R.SP. Recessus suprapinealis.

towards the temporal pole.

[graphic]
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