THE CEREBRUM. 695 



and divides into two cords or anterior pillars (crura) (Fig. 323, 17), which 

 pass in front of the anterior cerebral commissure, are inflected downwards 

 and backwards, in traversing the optic thalamus, on the sides of the middle 

 ventricle, and, finally, have their extremities confounded with the mammillary 

 process (corpus albicans). 



These two crura limit, in front, the foramen commune anterius or foramen 

 of Monro, over which the apex of the fornix is thrown across like an arch. 



The fornix is white throughout its whole extent, with a greyish tint 

 towards its summit. 



5. The Hippocampi. (Fig. 331, 4.) 



The hippocampus or cornu Ammonis (from its resemblance to a rarn's 

 horn, the crest of Jupiter Ammon), is an elongated projection, a veritable 

 internal convolution of the brain (is, in fact, the internal surface of the 

 gyrus fornicatus or convolution lying upon the corpus callosum, and which 

 terminates at the fissure of Sylvius) ; it occupies the floor of the anterior 

 region of the lateral ventricle, and is prolonged throughout its reflected por- 

 tion, whose curvature it exactly follows. Considered together, the two 

 hippocampi somewhat closely resemble the uterine cornua of the Cow. 



By their internal extremity, they are in contact with each other beneath 

 the middle portion of the fornix, and above the optic thalamus, which is sepa- 

 rated from them by the velum interpositum (Fig. 327, 9). Their external 

 extremity occupies, in the mastoid lobule, the cul-de-sac of the reflected 

 portion of the lateral ventricle. 



The central mass of this projection is formed by a nucleus of grey sub- 

 stance, covered superficially by a cortical layer of white a prolongation of 

 the posterior crura of the fornix. 



Towards the concave border of the hippocampus, this white lamina 

 offers a kind of wide hem, beneath which the choroid plexus passes ; this 

 hem constitutes a small curved band, like the cornu Ammonis, wider at its 

 middle part than at its extremities, and is named the corpus fimbriatum, or 

 tcenia hippocampus. 



6. The Corpora Striata. (Fig. 331, 7.) 



The corpus striatum is another projection on the floor of the cerebral 

 ventricle, occupying the anterior region of that cavity. 



This eminence is pyriform in shape, and obliquely elongated from behind 

 to before, and without to within. Its surface is smooth, and regularly convex. 

 Its base, or anterior extremity, corresponds to the anterior cascum of the 

 ventricle. The summit, or posterior extremity, disappears at the commence- 

 ment of the reflected portion of the ventricular cavity. Outwardly, the 

 corpus striatum is limited by a groove that forms the angle of union between 

 the floor and roof of the ventricle. Inwardly, it is separated from the optic 

 thalamus and cornu Ammonis by another sulcus, in which the choroid 

 plexus floats, and which is oblique inwards and forwards, and shows at the 

 bottom the tcenia semicircularis (Fig. 323, 13). This is a flattened white 

 cord, which disappears inwardly towards the foramen of Monro, and bends 

 outwards along the optic nerve to within about 3-8ths of an inch from the 

 chiasma ; in this way forming a sort of circular band around the anterior 

 extremity of the isthmus, beneath which all the fibres of the latter pass to 

 reach the cerebral hemispheres. 



The corpus striatum owes its name to its structure, being composed of a 



