THE NERVOUS SYSTEM. 181 
The Ventricles of the Brain.—The fourth ventricle is 
visible on the dorsal aspect of the medulla oblongata (Fig. 
g5). It is about three centimeters long by one centimeter 
wide, and les ventral to the cere- 
bellum. This ventricle is merely an ak 
expansion of the canalis centralis of 
the spinal cord. Its roof is very thin 
and consists of two portions, one of Oo ie 
which, the superior medullary velumt, a 
sometimes called the valve of Vieus- 4 
sens, covers the cranial half of the ven- ec 
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 
Fic. 94. D1acRAM OF 
VENTRICLES OF THE 
BRAIN VIEWED Dor- 
SALLY, 
ah, Anterior horn of 
the right lateral 
ventricle; as, aque- 
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 
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. 
fringes hanging from the roof of the 
ventricle on either side of the mid-line, form the choroid 
plexus, 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 iter, 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 muddle 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 
