THE BRAIN AND ITS MEMBRANES. 763 



Fifth Ventricle. The fifth ventricle was originally a part of the great longi- 

 tudinal fissure which separated the two hemisphere vesicles, but has become shut 

 off bv the union of the hemispheres in the formation of the corpus callosum and 

 the fornix. Its Avails are therefore formed by the median wall of the hemispheres, 

 and each consists of an internal layer of gray matter derived from the gray mat- 

 ter of the cortex and an external layer of white substance continuous with the 

 white matter of the cerebral hemispheres. This is lined on its external surface 

 by the ependyma of the corresponding lateral ventricle. The fifth ventricle is 

 not lined bv epithelium, but by a delicate layer of modified pia mater. It has no 

 connection "with any of the "regular" ventricles. 



The structures* of the floor of the descending cornu will now be con- 

 sidered. 



The hippocampus major, or cornu Ammonis (Figs. 445. 452. 456), so called from 

 its resemblance to a ram's horn, is a white eminence, of a curved elongated form, 

 extending throughout the entire length of the 

 floor of the middle horn of the lateral ventricle. 



At its lower extremity it becomes enlarged, and -^^^^^ 



presents a number of rounded elevations with ^^*^.^^^^P^l Eminent 



intervening depressions, which, from presenting C~ ^lyl/ coU(U - 



some resemblance to the paw of an animal, is 

 called the pes hippocampi. If a transverse sec- 

 tion is made through the hippocampus major 

 (Fig. 453). it will be seen that this eminence is 

 produced by the extension inward of the dentate Hipp./ 

 (hippocarnpal) fissure on the mesial aspect of the ' 

 temporal lobe. This fissure, like all the other fis- 

 sures of the hemisphere, is lined by a dipping in ^ ^ _ part of left desc cornu 

 and out a<?ain of the orav cortex; but, whereas in The cut surface looks forward. (Henie.) 



c - 7 



fissures the gray lining, after coming out, is 



continuous with that of an adjacent fissure, the gray lining of the hippocarnpal 

 ri*sun. after turning on itself, comes out and terminates in a free edge, forming a 

 notched ridge, the fascia dentata (Figs. 453, 454). The hippocampus is covered 



Corpus 



OaUoso-marffinal caUosutn 

 Jbwn 



Genu of tte 

 corp. call. 



Corpus 



^^ _^__ . fimbrialum 



Olfactory, " 

 lobe 



FIG. 451. The mesial, or internal surface of the right tiemispkere of a six months' foetus. (Schmidt.) 



on its ventricular surface by the lining membrane of the ventricle, beneath which 

 is a thin lamina of Avhite matter (alveus), which is continuous with the corpus 

 fimbriaturu of the fornix, and beneath this is the "gray matter" of the hippo- 

 campus i. e. the cortical lining of the hippocampal fissure, just described. This 

 gray matter is seen, on cross-section, to make a secondary turn which embraces a 

 slender process of white matter derived from the white lamina before it emerges 

 as the free edge. 



The corpus fimbriatum (Figs. 445. 454. 456) (tcenia hippocampi) is a narrow 

 white band situated immediately below the choroid plexus. It is the thin pro- 

 longation of the posterior pillar of the fornix, and is attached by its inner margin 



