668 



THE NERVOUS SYSTEM. 



first cerebral vesicle, which is not developed into nervous matter but is made up 

 only of ependyma covered by pia mater. The vessels of this portion of the highly 

 vascular pia mater become dilated and prolonged, and grow into the ventricle, 

 pushing the ependyma before them, and forming an irregular congeries of vessels, 

 apparently encroaching on the cavity of the lateral ventricle, but in reality being 

 external to it, because they are separated from it by the lining membrane of the 

 cavity, the ependyma. This vascular structure is the choroid plexus of the 

 body of the ventricle. 



The part of the choroid plexus seen in the descending cornu is formed in 

 exactly the same way, viz., by an ingrowth of the vessels of the pia mater into the 

 cavity, pushing the ependyma before it, at a part of the wall of the horn where 



FIG. 358. The fornix, velum interpositum, and middle or descending cornu of the lateral ventricle. 



there is a similar absence of nervous tissue and where it consists simply of pia 

 mater and ependyma in close contact. This portion lies between the corpus fim- 

 briatum in the floor and the taenia semicircularis in the roof of the descending 

 horn. This area, destitute of nervous matter, is continuous with the area in the 

 body of the ventricle, from which the choroid plexus of this region originated, and 

 in it the vessels of its pia mater increase, and, invaginating the ependyma, appear 

 in the descending horn as its choroid plexus. In the body of the ventricle the 

 choroid plexus is really the vascular fringed margin of the velum ; beyond the pos- 

 terior margin of the velum the plexus of the descending horn is continuous with 

 the pia mater on the surface of the gyrus hippocampi ; the two portions of the 

 plexus are, however, directly continuous with each other. The gap or cleft 

 through which the invagination of the pia mater takes place is called the trans- 

 verse fissure. 



