876 ANATOMY [NERVOUS SYSTEM which is in many parts ciliated, and which rests on a layer of neuroglia. This lining is continuous through the fora men 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 con tained in the cerebral ventricles, which, under some patho logical 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 FIG. 75. A deeper dissection of the lateral ventricle, nnd of the velum inter positum. a, under surface of corpus callosum, turned back ; b, b, posterior pillars of the fornix, turned back; e, c, anterior pillars of the fornix; d, velum intcrpositum and veins of Galen; e. fifth ventricle; /, /, corpus striatum; 0, ?, tasnia Bemicircularis ; h, h, optic thalamus; i, choroid plexus ; /, taania hippocampi; m, hippocampus major in descending cornu; n, hippocampus minor; o, eminentia collateralis. by a transverse incision, and the posterior half of this structure be turned back, 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 ventri cles 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 ver tical mesial space, the fifth ventricle. If the septum be now removed, the anterior part of the fornix is brought into view. The fornix or arch is an arch-shaped band of nerve fibres extending in the antero-posterior direction. Its ante rior end forms the anterior piers or pillars of the arch, its posterior end the posterior piers or pillars, whilst the inter mediate body of the fornix forms the summit or 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 conjoined to form the body; but in front of the body 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 then enter the substance of the optic thalamus. Behind the body the two halves diverge much more from each other, and form the posterior pillars; each of which curves downwards and outwards into the descending cornu of the ventricle, and, under the name of tcenia hippocampi, forms the free border of the hippocampus major. If the body of the fornix be now divided by a transverse incision, its anterior part thrown forwards, and its posterior part backwards, the great trans verse 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 interior of the hemispheres through the great transverse fissure. It is triangular in shape; its base is in 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 tho 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, which supply the cor pora striata, optic thalami, and corpora quadrigemina; and the blood from these bodies is returned by small veins, which join to form the veins of Galen (Fig. 75). These veins pass along the centre of the velum, and, as is shown in Fig. 63, open into the straight sinus. If the velum interpositum be now carefully raised from before backwards, the optic thalami, third ventricle, pineal gland, and corpora quadri gemina are exposed. The optic thalamus is a large, somewhat ovoid body situated behind the corpus striatum, and above the cms 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, placed one on each side of the optic tract, and named respectively corpus geniculatum internum and externum. The third ventricle is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of fornix; its floor, by the pons Tarini, corpora albicantia, tuber cinereum, infundibulum, and optic commissure ; its anterior bound ary, by the anterior pillars of the fornix, anterior commis sure, 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; but in taking the brain out of the cranial cavity this commissure is usually more or less torn through, and the cavity is consequently enlarged. Imme diately 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 entering the two corpora striata. The pineal body is a reddish cone-shaped body, enveloped by the velum interpositum, nnd situated upon the more anterior pair of the corpora quadrigemina. From its broad anterior end two white bands, the peduncles of the pineal body, pass forwards, one on the inner side of each optio thalamus. Each peduncle joins, along with the ta^nia semicircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of a vascular stroma of connective tissue, in the meshes of which lymphoid cells arc contained. Branched corpuscles are
also found not unlike nerve cells. Amylaceous and gritty
