THE BRAIN AND ITS MEMBRANES. 



f69 



Choroid Plexus of Descending Cornu. The epithelial inner wall, just described, 

 is invaginated by and closely invests a fringe-like margin of pia mater (Fig. 461), 

 which apparently passes into the ventricle, turns on itself, and passes out again, 

 but is everywhere covered, toward the cavity of the cornu, by the latter's now 

 greatly convoluted inner epithelial Avail. This is the choroid plexus of the 

 descending cornu, and when seen from above it lies over the hippocampus major 

 and conceals it from view, as well as the corpus fimbriatum. 



The two layers of pia mater, of which the margins, covered by epithelium, 

 make up the choroid plexus, are in continuity with the two layers of the velum 

 interpositum. whose margins, also covered by epithelium, form the choroid plexuses 

 of the bodies of the ventricles. But the upper layer of the velum is continuous 



FIG. 461. The same as the two preceding figures. The velum interpositum and choroid plexuses are now com- 

 plete. In the roof of the descending cornu are seen the prolongations of ta?nia semicircularis and caudate 

 nucleus. (B. B. G.) 



with the lower layer of the choroid plexus of the cornu, and, vice versd, this rela- 

 tion being due to the bending downward, forward, and inward of the temporal 

 lobe and the descending cornu. This relation may perhaps be better appreciated 

 by tracing these layers separately, thus : 



The loicer layer of the "choroid plexus" of the descending cornu, if traced 

 out of the cornu /. e. toward the median line passes, at its lower part, right 

 around the under surface of the temporal lobe ; if traced at its upper part i. e. 

 at the curve of junction between this cornu and body of ventricle this same 

 layer will be found to bend sharply forward on itself and to come forward under 

 the edge of the now superiorly placed fornix, and be continuous with the upper 

 layer of the velum. Posteriorly and externally, this layer is in continuity with 

 the pia mater covering the under surfaces of the occipital and temporal lobes. 



The upper layer of the " choroid plexus " of the descending cornu, if traced 

 in the same manner i. e. at two levels is found, at the lower level, to be con- 

 tinuous with the pia mater covering the crustse of the mid-brain ; at the upper 

 level it also bends sharply forward, comes forward under the fornix, and is con- 



49 



