THE MAMMALIAN BRAIN 
79 
make a median sagittal section through the entire brain by a 
single carefully directed stroke of the wet blade of a long 
thin sharp knife. Not more than half of the supply of 
specimens should be used for this purpose, and usually fewer 
will suffice since two students may conveniently study one good 
section. 
• 
The Cavities of the Brain. —The brain is a hollow organ, the 
walls of which have undergone repeated folding and thickening, 
and the cavity of which is continuous with that of the spinal cord, 
the canalis centralis. The inner surface of the cavity, covered by 
a smooth epithelium, the ependyma, should be carefully distin¬ 
guished from the cut surfaces of its walls. The cavities are known 
as ventricles, four in number and disposed as follows: The first 
and second (to be demonstrated by later dissection) in the two 
cerebral hemispheres, i.e., in the telencephalon, the third in the 
diencephalon, connected with the first and second by a channel 
known as the foramen interventriculare. The third ventricle 
is connected with the fourth ventricle, which is located in the 
rhombencephalon, by the aquasductus cerebri, which passes 
through the mesencephalon. The first and second ventricles are 
separated from each other by a thin median partition, the septum 
pellucidum, triangular in outline, and bounded above by the cor¬ 
pus callosum and below by a curved longitudinal bundle of fibers 
known as the body of the fornix. The anterior boundary of the 
third ventricle appears as a thin lamina, the lamina terminalis, 
extending ventrally from below the anterior end of the fornix to 
the optic chiasma. The roof of the anterior part of the third 
ventricle is very thin and is known as the epithelial chorioid 
lamina, through which the anterior chorioid plexus (blood vessels) 
pushes its way to reach the inner surface of the third ventricle. 
The extensive, thin roof to the fourth ventricle is revealed by 
carefully lifting up from contact with it, the overhanging thick¬ 
ened portion which forms the cerebellum. It consists of the 
anterior medullary velum, which lies anterior to the junction of 
the cerebellum with the medulla, and the posterior medullary 
velum, which lies posterior to the cerebellum. Through the 
latter velum the posterior chorioid plexus reaches the interior 
of the fourth ventricle. 
