792 THE CENTRAL AXTS OF THE NERVOUS SYSTEM. 



them by the velum interpositiun (Fig. 429, 9). Their external extremity occupies, 

 in the sphenoidal lobe, the cul-de-sac of the reflected portion of the lateral ventricle. 



The central mass of this projection is formed of a nucleus of grey substance, 

 covered on both faces by a layer of white substance. The layer that covers its 

 inferior face is named the subiculum, and that which extends over its surface is 

 designated the alveus ; it is a kind of prolongation of the posterior pillars of the 

 fornix. 



Towards the concave border of the hippocampus, this white layer offers a 

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

 a small curved band, like the cornu Ammonis, wider in its middle part than at 

 its extremities, and is named the corpus fimbriatum, or tcBnia hippocampus. 



The grey layer comprised between the two white bands also makes a slight 

 projection at the inner border of the hippocampus, and forms the fascia dentata} 



6. The Corpora Striata (Fig. 435, 7). 



The corpus striatum is a mass of grey matter interposed on the course of the 

 crura cerebri. It includes the entire thickness of the floor of the lateral ventricle, 

 and projects outwards, on the lower face of the hemisphere, between the two 

 roots of the olfactory lobule. 



The corpus striatum owes its name to its structure : the thick nucleus of 

 grey matter composing it is, in fact, traversed by white fibres from the crura 

 cerebri, which pass into the hemispheres ; these fibres appear at several points 

 in the form of sharply defined white strife. 



It is divided into two portions by these fibres, which collectively constitute 

 the internal capsule (Fig. 436, ci). The external portion is the extra-ventricular 

 nucleus of the corpus striatum, so named because of its position, and lenticular 

 nucleus, in consequence of its shape ; just as the internal portion is most 

 frequently designated the iiitra-ventricular nucleus of the corpus striatuyn, or 

 caudate nucleus (Fig. 436). The caudate or intra- ventricular nucleus occupies the 

 anterior region of the lateral ventricle (Fig. 435, 7) 



This eminence is pyriform in shape, and obliquely elongated forward and 

 inward. Its surface is smooth, and regularly convex. Its base, or anterior 

 extremity, corresponds to the anterior cid-de-sac of the ventricle. The summit, 

 or posterior extremity, disappears at the commencement of the reflected portion 

 of the yentricular 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 

 groove, in which the choroid plexus floats, and which is oblique inwards and 

 forwards, and shows at the bottom the tcBnia semicircularis (Fig. 432, 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 | of an inch 

 from the optic commissure ; in this way it forms a kind of circular band around 

 the anterior extremity of the isthmus, beneath which all the fibres of the latter 

 pass to reach the cerebral hemispheres. 



7. The Velum Interpositum and Choroid Plexus (Fig. 435, 6). 



The velum inteipositum {velum vasculosum, tela choroidea) is a vascular 

 expansion derived from the pia mater, which penetrates the brain by the great 



' Sabaticr, taking comparative anatomy and embryology as his guide, regarded the hem of 

 the hippocampus as a ganglion spread along the origin of tiie optic nerve. 



