ANATOMY OF THE NERVOUS SYSTEM. 



169 



which has a very thin roof, lapped over by the cerebellum. 

 From the front of the fourth ventricle rims a narrow pas- 

 sage (aqueduct of sylvius or iter) which enters another dila- 

 tation, 3, Fig. 75, lying in the middle line near the under 

 side of the fore-brain and known as the third ventricle. 

 From the third ventricle two apertures (the foramens of 

 Monro), one of which is partly seen at m in the diagram, 

 lead into the first and second, or lateral ventricles, one of 

 which lies in each of the cerebral hemispheres. The front 

 ends of these two ventricles are seen in the vertical trans- 

 verse section of the brain represented in Fig. 76. 



VI 



FIG. 76. A vertical section across the cerebral hemispheres taken in front of the 

 fifth ventricle. Cci 2 , anterior part of corpus callopum ; VI, the anterior end of the 

 right lateral ventricle: the gray mass on its exterior is the front end of the corpus 

 striatum. On the left side the superficial gray matter covering the convolutions 

 is shaded. 



The ventricles contain a small amount of cerebro-spinal 

 liquid, and are lined by epithelium which is ciliated in early 

 life. Part of the posterior wall of the third ventricle is ex- 

 tremely thin, consisting of little but this epithelium sup- 

 ported by a thin layer of pia mater: this part is pushed in or 

 doubled into the cavity of the ventricle in the form of a 

 triangular membrane, the velum interpositum, which lies 

 beneath the fornix and sends offshoots into the lateral ven- 

 tricles. Between the upper and lower layers of the indupli- 

 cated velum interpositum arteries enter and there break up 

 into plexuses the choroid plexuses covered everywhere by 



