768 



THE NERVOUS SYSTEM. 



Epithelial Floor of the Body of the Ventricle. The margin of the velum inter- 

 positum thus formed is necessarily situated between the under surface of the floor 

 of the body of lateral ventricle above and the upper surface of optic thalamus 

 below, but it does not reach out over all of this surface, but lies only on its inner 

 half, as already explained. Hence a portion of the under surface of the floor of 

 the ventricle must rest on the outer half of the upper surface of the thalamus. 

 Now, this portion of the floor, together with that immediately superjacent to the 

 margin of the velum interpositum, becomes reduced to a layer of epithelium 

 which stretches from the edge of the fornix over to the tcenia semicircularis. This 

 epithelium is continuous with that lining the ventricle both at the edge of the 

 fornix and at the tgenia. As it passes over the fringe-like margin of the velum 

 interpositum it invests all its processes, and thus forms the true choroid plexus. 

 As it passes over the optic thalamus it has ependyma beneath it, as also where it 



'ody of Ventricle 



Fornix 



White Matter 

 (Temporal Lobe) 



Inner Wall 

 of 



FIG. 460. The same as preceding figure, but at a supposedly later stage of development. (B. B. G.) 



covers tsenia semicircularis, caudate nucleus, under surface of corpus callosum, 

 ventricular aspect of septum lucidum, and upper surface of corresponding half 

 of fornix. 



Epithelial Inner Wall of Descending Cornu. The entire inner wall of this 

 cornu is reduced to a layer of epithelium. It is, morphologically, the continua- 

 tion of the epithelium forming part of the floor of the body of the ventricle just 

 described, and it stretches between the same structures, or rather their prolonga- 

 tions i. e. tcenia semicircularis in roof of descending cornu and corpus fimbriatum 

 in floor (Figs. 460 and 461). In the region of transition from body to descend- 

 ing cornu, just at the curve, the epithelium curves downward also, and stretches, 

 now, between edge of posterior pillar of fornix (posterior part) across, on the 

 rounded pulvinar of the optic thalamus, to the curved part of the tcenia, which 

 is immediately external to and lies against the outer aspect of the pulvinar. 

 Hence this part of the epithelium is, strictly speaking, a portion of the roof of 

 the descending cornu (see p. 758). Just beyond this point the epithelium assumes 

 the mesial position and becomes the regular inner wall of the cornu. 



