144 PHILOSOPHY OF ZOOLOGY. 
a transverse hard and white membrane termed Corpus callo- 
sum, appears, of considerable thickness, convex, with its an- 
terior and posterior extremities incurved. At each of these 
extremities, the hemispheres are united by a medullary cord, 
termed the anterior and posterior commissures. Under- 
neath the corpus callosum, is the less dense partition, term- 
ed Septum lucidum, the inferior arched side of which is 
termed the Fornix. On each side of this division, the sub- 
stance of the brain admits readily of a separation, exhibit- 
ing two cavities, termed the Lateral Ventricles of the brain, 
which are said to communicate with each other, near the 
middle of the fornix. In the lower and interior parts of 
each ventricle, there is a curved medullary prolongation, 
termed Cornu Ammonis. In the anterior part of each ven- 
tricle, there is a pyriform eminence, of a cineritious colour, 
and striated when divided. These are termed Corpora 
striata. Behind these, lie two other eminences, of a whiter 
colour, united by a soft transverse plate of medullary mat- 
ter, forming what are termed T’halami nervorum opticorum. 
Between these, there is another cavity, termed the Third 
Ventricle of the brain. The communications between the 
different ventricles, and the fluids which they contain, are 
still subjects. of controversy. Immediately behind the tha- 
lami, lies the Pineal Gland, remarkable for containing small 
concretions, like sand, consisting of phosphate of lime. Be- 
neath this gland, are situated four medullary eminences, 
termed Corpora quadrigemina, the two superior and ante- 
rior of which are called nates ; the inferior and posterior, 
testes. In each lateral ventricle, there is a plexus of mi- 
nute arteries and veins, which are expanded upon the op- 
tic thalami, the pineal gland, and corpora quadrigemina. 
The cEREBELLUM occupies the inferior cavities of the 
occipital bone, and is covered by the posterior lobes of the 
cerebrum. Its surface is marked by transverse furrows, 
