THE CEREBRAL HEMISPHERES. 



939 



The taenia semicircularis, lying along the ental border of the ventricular surface 

 of the caudate nucleus, is a small band of white fibres arching from the amygdaline 

 nucleus (near the temporal pole) to the Anterior perforated substance. The 

 entrance of a part of the thalamus into the formation of the floor of the lateral 

 ventricle is apparent enough, but morphologically it should be strictly excluded 

 therefrom. The thalamus is in no way formed from the parietes of the secondary 

 fore-brain vesicles (telencephalon), for it is, in fact, excluded by a layer of ependyma 

 (lamina affixa) reflected onto, and often separable from, the surface of the thala- 

 mus, so that it appears as a constituent of the floor because of the transparency of 

 the ependymal sheet. The choroid plexus of the lateral ventricle is a richly 

 vascular imagination over which the ependyma is continuous to again become 

 reflected onto the fornix along its sharp edge. A reference to Fig. 666, show- 

 ing the topographical relations of these structures in a frontal section, may be of 



The cavity is thence continued ventrolaterad in a bold sweep to become ex- 

 panded as an obliquely pyramidal space of a somewhat triangular outline on 

 section, and placed subjacent to the parietal lobe the trigonum ventriculi. A 

 conspicuous feature in its floor is the collateral eminence, correlated with the 

 collateral fissure. From the trigonum, the most capacious part of the lateral 

 ventricle, the cavity is prolonged in opposite directions as the middle and pos- 

 terior cornua. 



Tail of caudate nucleus 



Choroid plexus 



Einthelial lining of ventricl& 



Pia mater 



Fimbria 

 Fintbrio-dentate. 

 fissure 



Aheus 



FIG. 691. Coronal section of descending horn of the lateral ventricle. (Diagrammatic.) 



The middle horn or medicornu (cornu inferius] is a prolongation of the ventricular 

 cavity, from its trigone toward the temporal pole, which pursues a curved course 

 with its convexity directed ventrolaterad (Figs. 689 and 690), corresponding 

 to the curved contour of the temporal lobe, and situated at a depth of about 3 cm. 

 from its lateral surface as well as from the temporal pole. The roof is formed by 

 (a) the tapetum of the corpus callosum; (6) the cauda (tail) of the caudate nucleus; 

 (c) the taenia semicircularis. The medial wall is principally composed of the 

 hippocampus, a prominent welt-like eminence bulging into the cavity, largely 

 filling it, and produced by the hippocampal fissure. The hippocampus nearly 

 conceals from view the actual floor, which is of variable extent in different brains 

 and usually marked by an extension of the collateral eminence previously described. 

 Surmounting the corrugated hippocampal formation and projecting slightly 

 into the cavity, is the fimbria, and from its sharp edge the ventricular ependyma 



