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backwards from the superior and posterior part (i.e. the convexity) of the primitive cavity.

Cornu Anterius.-The anterior horn forms the foremost part of the cavity, and extends in a forward and lateral direction in the frontal lobe. When seen in frontal section (Fig. 562) it presents a triangular outline, the floor sloping upwards and laterally to meet the roof at an acute angle. It is bounded in front by the posterior surface of the genu of the corpus callosum; the roof also is formed by the

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FIG. 563.-DISSECTION, to show the fornix and lateral ventricles; the body of the corpus callosum

has been turned over to the left.

corpus callosum. The medial wall, which is vertical, is formed by the septum pellucidum; whilst the sloping floor presents a marked elevation or bulging, viz., the smooth, rounded, and prominent extremity of the pear-shaped caudate


Pars Centralis. The central part or body of the cavity is likewise roofed by the corpus callosum. On the medial side it is bounded by the posterior part of the septum pellucidum which attaches the fornix to the inferior surface of the corpus callosum. On the lateral side it is closed, as in the case of the anterior horn, by the meeting of the floor and the roof of the cavity. On the floor a number of important

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objects may be recognised. From the lateral to the medial side these are met in the following order: (1) the caudate nucleus; (2) a groove which extends obliquely from before backwards and laterally between the caudate nucleus and the thalamus, in which are placed the vena terminalis and a white band called the stria terminalis; (3) a portion of the superior surface of the thalamus; (4) the chorioid plexus; (5) the thin, sharp edge of the fornix (Fig. 564).

The caudate nucleus narrows rapidly as it proceeds backwards on the lateral part of the floor of the lateral ventricle. The vena terminalis (O.T. vein of the corpus striatum) is covered over by ependyma. It joins the vena cerebri interna close to the foramen interventriculare. The connexions of the stria terminalis will be dealt with later. The portion of the superior surface of the thalamus which appears in the floor of the ventricle is in great part hidden by the chorioid plexus, which lies upon it. The lamina chorioidea is an epithelial fringe which is attached to the sharp edge of the fornix superiorly and after surrounding a rich vascular fold of pia mater becomes fixed to the superior surface of the thalamus. The vascular fold is the chorioid plexus. In front it is continuous, in the interventricular foramen, with the corresponding chorioid plexus of the third ventricle (Fig. 560), whilst behind, it is carried into the inferior horn of the ventricle. Although the chorioid plexus has all the appearance of lying free within the ventricle, it must be borne in mind that it is invested by the epithelial

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chorioidal lamina which represents a portion of the hemisphere wall and excludes it from the cavity.

Cornu Posterius.-The posterior horn is an elongated diverticulum carried backwards into the occipital lobe from the posterior end of the ventricle. It tapers to a point and describes a gentle curve, the convexity of which is directed laterally. The roof and lateral wall of this portion of the ventricular cavity are formed by the tapetum of the corpus callosum. In frontal sections through the occipital lobe this is seen as a thin but distinct layer of white fibres, which lies immediately lateral to the ependyma and to the medial side of a much larger strand of fibres in the medullary substance of the occipital lobe, viz., the optic radiation.

On the medial wall two elongated curved elevations may be observed. The uppermost of these is termed the bulb of the cornu (bulbus cornu posterioris), and is produced by the fibres of the radiation of the corpus callosum as they curve abruptly backwards from the lower part of the splenium of the corpus callosum into the occipital lobe. Below this is the elevation known as the calcar. It varies greatly in size in different brains, and is caused by an infolding of the ventricular wall in correspondence with the anterior part of the calcarine sulcus on the exterior of the hemisphere. It may come into contact with and adhere to the lateral wall of the ventricle in a part or even the whole of its extent.

Cornu Inferius. The inferior horn is the continuation of the cavity into the temporal region. At first directed backwards and laterally, the inferior horn suddenly sinks downwards behind the thalamus into the temporal region, in the

centre of which it takes a curved course forwards and medially to a point about an inch behind the extremity of the temporal pole.

In the angle between the diverging posterior and descending horns the cavity of the ventricle presents an expansion of a somewhat triangular shape. To this the name of trigonum collaterale is sometimes given.

The roof of the inferior horn is formed for the most part by the tapetum of the corpus callosum. At the extremity of the horn the roof presents a bulging into the cavity. This is produced by a collection of gray matter termed the amygdaloid nucleus. The stria terminalis and the attenuated tail of the caudate nucleus are both prolonged into the inferior horn and are carried forwards, in its roof, to the amygdaloid nucleus.

On the floor of the inferior horn the following structures are seen: (1) hippocampus; (2) the chorioid plexus; (3) the fimbria; and (4) the eminentia. collateralis.

The hippocampus is for the most part covered by the chorioid plexus of the lateral ventricle. If this is detached a fissure appears between the fimbria and the roof of the ventricular horn. This is the chorioid fissure. It appears at a very Splenium of corpus callosum

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early date in the development of the cerebral hemisphere, and takes an arcuate course round the posterior end of the thalamus. In the region of the pars centralis of the lateral ventricle it extends as far forwards as the foramen interventriculare and is formed by the involution of an epithelial part of the wall of the ventricle over the chorioid plexus (p. 622). In the region of the inferior horn, when the chorioid plexus, with the involuted epithelial layer which covers it, is withdrawn, the chorioid fissure is converted into an artificial gap which leads directly into this part of the ventricular cavity.

The chorioid plexus is a convoluted system of blood-vessels in connexion with a fold of pia mater, which is prolonged into the inferior horn of the lateral ventricle. It lies on the surface of the hippocampus and is continuous, behind the posterior part of the thalamus, with the chorioid plexus in the pars centralis of the lateral ventricle. But it must not be supposed that the chorioid plexus lies free in the ventricular cavity. It is clothed in the most intimate manner by an epithelial layer, which represents the medial wall of the inferior horn involuted into the cavity over the chorioid plexus. The ventricle, therefore, opens on the surface only through the chorioid fissure when this thin epithelial layer is torn away by the withdrawal of the chorioid plexus. From the above, it will be under

stood that the arcuate chorioid fissure, throughout its whole length (viz., from the interventricular foramen to the extremity of the inferior horn of the lateral ventricle), is formed by the


involution of the roof and a portion of the wall of the hemisphere which remains epithelial. In the central part of the ventricle this layer is attached, on the one hand, to the sharp margin of the fornix, and on the other to the superior surface of the thalamus; in the inferior horn it is attached, in like manner, to the edge of the fimbria hippocampi or crus of the fornix,

whilst, above, it joins the roof FORNIX HO
of this portion of the ventricle
along the line of the stria
terminalis (Fig. 564).

The eminentia collateralis shows very great differences in its degree of development. The trigonum collaterale is a smooth elevation in the floor of the ventricle, in the interval which is left between the calcar avis and the hippocampus as they diverge one from the other.


Bulb of the cornu

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Under this heading are included certain masses of F. gray matter more or less com

pletely embedded in the white medullary substance of the hemisphere, and which are developed in its wall. They compose the caudate and lentiform nuclei, which together form the corpus striatum, and the amygdaloid nucleus.

The nucleus caudatus bulges into the lateral ventricle. It is a piriform, highly arched mass of gray matter, which presents a thick, swollen head, or anterior extremity, and a long, attenuated tail. The head projects into the anterior horn of the lateral ventricle, whilst its narrower part is prolonged laterally and posteriorly in the floor of the ventricle, where it is separated from the thalamus by the stria terminalis. Finally, its tail curves downwards with a bold sweep and enters the inferior horn of the lateral ventricle. In the roof of this horn it is prolonged forwards to the amygdaloid nucleus, the lower part of which it joins. The caudate nucleus thus presents a free ventricular surface, covered with ependyma, and a deep surface embedded in the white substance of the cerebral hemisphere, and for the most part related to the internal capsule.

Owing to its arched form it follows that, in horizontal sections through the cerebral hemisphere below a particular level, it is cut at two points, and both the head and the tail appear on the field of the section (Fig. 567). In frontal sections behind the amygdaloid nucleus, it is also divided at two places.

The anterior extremity of the head of the caudate nucleus coincides very nearly with that of the anterior horn of the lateral ventricle. In the region of the substantia perforata anterior, the head of the caudate nucleus gains the surface and its gray matter becomes continuous with that of the cerebral cortex.

The nucleus lentiformis lies on the lateral side of the caudate nucleus and thalamus, and is for the most part embedded within the white medullary substance of the cerebral hemisphere. It does not extend either so far forwards or so far backwards as the caudate nucleus. Indeed, it presents a very close correspondence in point of extent with the insula on the surface. When seen in horizontal section, it presents a shape similar to that of a biconvex lens.

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capsule with
cularis fibres

ansa lenti

in blue

Its medial surface bulges

more than the lateral surface, and its point of highest convexity is placed opposite the stria terminalis and the interval between the caudate nucleus and the thalamus. In frontal section the appearance presented by the lentiform nucleus differs very much in different planes of section. Fig. 568 represents a section through its anterior portion. Here it is semilunar or crescentic in outline and is directly continuous below

with the head of

-Optic radia- the caudate nuction passing leus; above, also, white line in it is intimately



connected with the caudate nucleus by bands of gray matter, which pass between the two nuclei and break

up the


matter of the an

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or barred appearance, which is presented by such a section as this, that the term corpus striatum is applied to the two nuclei. In the region of the substantia perforata anterior both nuclei reach the surface and become continuous with the


When a section is made in a plane further back (e.g. immediately posterior to the anterior commissure, as in Fig. 569) the divided lentiform nucleus assumes an altogether different shape, and is seen to be completely cut off from the caudate nucleus by the internal capsule. It is now triangular or wedge-shaped. Its base is turned towards the insula and is in direct relation to a thin lamina of white matter, termed the external capsule. Its medial surface is oblique and is applied to the internal capsule, whilst its inferior surface is horizontal and is directed downwards towards the base of the brain. But, further, two white lamina, the

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