134 ESSENTIALS OF ZOOLOGY 



The anterior division is the telencephalon, which includes the 

 olfactory lobes and cerebrum. The former are paired lobes at the 

 most anterior end. They are well developed and serve as centers of 

 the sense of smell which is quite specialized in the rat. The cerebrum 

 is divided longitudinally into two hemispheres one on each side of 

 the dorsal median fissure. There are no deep furrows or convolutions 

 in the surface of the cerebrum as is the case in the cat, man, or a 

 number of other mammals. The cerebrum is relatively large and has 

 spread until the two middle divisions of the brain have been covered 

 by it. Between the two hemispheres is a broad band of fibers, the 

 corpus callosum, which connects them. The fornix, anterior commis- 

 sure, and posterior commissure are other bundles of nerve fibers con- 

 necting the two hemispheres. Thus there is coordination of function 

 in the two halves of the cerebrum. This division of the brain serves 

 as the center of voluntary control, correlations, and many associations. 



The diencephalon or thalamus which is in the anterior portion of 

 the brain stem and is covered by the cerebrum forms the principal 

 connections between the cerebrum and other parts of the brain. In 

 particular there is a relationship between the olfactory lobes and this 

 part. The diencephalon is another important center of the sense of 

 smell. 



The mesencephalon, or midbrain, is located just posterior to the 

 preceding division and is also covered by the posterior portion of the 

 cerebral hemispheres. On its dorsal side are located the corpora 

 quadrigemina. There are four of these prominences, and they are 

 homologous to the paired optic lobes of the frog and other simpler 

 vertebrates. The ventral portion of the midbrain consists primarily 

 of two large bundles of nerve fibers called the cerebral peduncles. 

 The fibers of these make the numerous connections between the cere- 

 brum and medulla, which in turn leads to the spinal cord. There 

 are visual and auditory centers located in the midbrain. 



The cerebellum is the convoluted portion of the brain which ap- 

 pears externally, just posterior to the posterior margins of the cere- 

 bral hemispheres. Its position is just posterior to the midbrain also. 

 Its median lobe is the vermis, and at the sides of this are the right 

 and left hemispheres. The large bundles of fibers extending to the 

 ventral side of the brain at this level constitute the pons and repre- 

 sent the only portion of the cerebellum to appear on the ventral side. 

 This division of the brain serves as a center of coordinated movement 

 and is particularly associated with balance. 



