766 



THE NERVOUS SYSTEM. 



The choroid plexuses (Figs. 457, 458) of each lateral ventricle are two in 

 number, one in the floor of the body, and the other in the descending cornu. 

 Each is a vascular fringe-like membrane with a free edge looking toward the 

 ventricular cavity, and an attached margin which is continuous with two layers 

 of pia mater. 



The choroid plexus of the body of the ventricle is, as before stated, the thick- 

 ened, convoluted side of the velum interpositum, which is made up of two layers 

 of pia mater. The reasons for the presence of two layers in the velum interpo- 

 situm, as well as for their continuity with one another at their free margins, and 



Gcnu of corp. call. 



th ventricle 



Caud. nucl. 



- Ant pillar of fornix 

 Velum interpos. 



Optic thalaiuus 



Pes hippocampus 



Pineal gland 

 Ant. corp. q. 

 Chor, pi. of 3d v 



Vena Galeni 



Posterior cornu 



Pia mater 



FIG. 458. Choroid plexus of body of ventricle and of descending cornu of left side. The velum interposi- 

 tum is split on the left side, showing choroid plexus of third ventricle. (Henle.) 



for the fact of the existence of such free margins, must be sought for in the method 

 of development. 



In the brief account given of the development of the hemisphere vesicles no 

 mention was made of the pia mater. But it must be understood, of course, that 

 the pia mater covers the entire brain-tube, and takes part in, and adapts itself to, 

 all the diiferent changes in shape and position which the various portions of the 

 brain undergo. 



We thus have, at a certain point of development, three " tubes " of pia mater 

 one encircling the inter-brain, and one investing each hemisphere vesicle (see 

 Fig. 459). As the latter approaches its fellow it also grows in toward the inter- 

 brain. Finally, as already described, actual adhesions take place between the 

 mesial aspects of each hemisphere above the inter-brain, and between that portion 

 of the hemisphere which lies external to the inter-brain and the side of the inter- 

 brain (optic thalamus) ; while the under surfaces of the hemispheres (corpus cal- 

 losum and fornix) merely rest on, and are not adherent to, the superior surface of 

 the inter-brain. The effect on the pia mater (see Figs. 460, 461) mesially and 



