THE NERVOUS SYSTEM. 



181 



The Ventricles of the Brain. The fourth ventricle is 

 visible on the dorsal aspect of the medulla oblongata (Eig.. 

 95). It is about three centimeters long by one centimeter 

 wide, and lies ventral to the cere- 

 bellum. This ventricle is merely an 

 expansion of the canalis centralis of 

 the spinal cord. Its roof is very thin 

 and consists of two portions, one of 

 which, the superior medullary velum, 

 sometimes called the valve of Vieus- 

 sens, covers the cranial half of the ven- 

 tricle; the other portion is the inferior 

 medullary velum, lying over the caudal 

 half. The latter velum is composed of 

 a fold of pia mater tucked in between 

 the cerebellum and medulla, in addition 

 to a layer of epithelial cells on the ven- 

 tricular aspect of the pia mater. Some 

 nervous matter in addition to the pia 

 mater and epithelium forms the supe- 

 rior velum. Two longitudinal vascular 

 fringes hanging from the roof of the 

 ventricle on either side of the mid-line, form the choroid 

 ple.rus, which is merely a network of blood-vessels carried 

 by a reflected portion of the pia mater. 



Craniad the fourth ventricle is continued as a small canal, 

 the tier, or aqueduct of Sylvius, which lies ventrad to the 

 corpora quadrigemina and opens into the third ventricle 

 (Fig. 95). The latter is a narrow, vertical, cleft-like space 

 between the optic thalami. The two thalami are united by 

 the soft or middle commissure, better designated as the 

 massa intermedia, extending through the ventricle. Unless 

 this ventricle has been injected with a starch mass through 

 the infundibulum before the brain was hardened, its cavity 



FIG. 94. DIAGRAM OF 

 VENTRICLES OF THE 

 BRAIN VIEWED DOR- 

 SALLY. 



ah, Anterior horn of 

 the right lateral 

 ventricle ; as, aque- 

 duct of Sylvius; cc, 

 canalis centralis of 

 the spinal cord; 3 

 and 4, third and 

 fourth ventricles ; 

 m, foramen of 

 Monro; mi, middle 

 horn or cornu of 

 the ventricle. 



