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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
FIG. 557.-THREE STAGES IN THE DEVELOPMENT OF THE CORPUS CALLOSUM.
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 stria 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 dentata 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
Sulcus sagittalis cune
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
FIG. 559. THE CORPUS CALLOSUM, exposed from above and the right half dissected,
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
In frontal sections through the occipital and posterior temporal regions the tapetum stands out very distinctly (Fig. 559, p. 631; see also Figs. 565, p. 636, and 567, p. 638).
Septum Pellucidum.-The septum pellucidum is a thin vertical partition which intervenes between the two lateral ventricles. It is triangular in shape, and posteriorly it is prolonged backwards for a variable distance between the body of the corpus callosum and the fornix, to both of which it is attached. In front it occupies the gap behind the genu of the corpus callosum, whilst below, in the narrow interval between the posterior edge of the rostrum of the corpus callosum and the fornix, it is prolonged downwards in the paraterminal body towards the base of the brain. The septum pellucidum is composed of two thin laminæ in apposition with each other in the median plane (Fig. 562; Fig. 564, p. 635).
Cavum Septi Pellucidi.-This name is applied to the median cleft between the
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 ventricle are of common occurrence. 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
towards the temporal pole.
The early foetal lateral ventricle is very
capacious and presents an arched or semilunar form. 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. It comes into existence as a diverticulum or elon
FIG. 562.-FRONTAL SECTION THROUGH THE CEREBRAL HEMISPHERES So as to cut through the anterior horns of the lateral ventricles, through which the central part of the ventricles, the columns of the fornix, and the interventricular gated
foramina can be seen.