IV. VERTEBRATA: MAMMALIA. 



623 



and fishes, the cerebellum (IV) is differentiated into a median 

 verrnis and lateral cerebellar hemispheres. In the cerebrum the 

 mantle comes first into consideration. Its frontal lobes grow for- 

 wards over the olfactory lobes, which consequently lie farther and 

 farther back on the lower surface. The temporal lobes extend 

 right and left over the optic lobes and down to the floor of the 

 cranium ; the occipital lobes cover successively the mid brain, cere- 

 bellum, and medulla oblongata. Since the greatest increase 

 of intelligence lies within the mammals, the cerebra may be 

 arranged in an ascending series. In the monotremes, marsupials, 

 insectivora, and rodents (fig. 649, A) the olfactory lobes are 



A B C 



-to 

 x. Y-^a 



FIG. 649. A, brain of rabbit (after Gegenbaur); J3, of fish otter. <7, of pavian 

 monkey (after Leuret and Gratiolet). I, cerebrum; III, optic lobes; IT 7 , cerebel- 

 lum; V, medulla oblongata; to, olfactory lobes. 



visible in front, usually the mid brain behind (/// ). In the lemurs, 

 carnivores (fig. 649, B), and ungulates the olfactory lobes are 

 completely, the cerebellum partly, covered. In man and the 

 anthropoid apes, on removing the roof of the skull, only the two 

 cerebral hemispheres are visible, all other parts being more or less 

 completely covered. 



Further, it is to be noted that in the first group the surface of 

 the cerebrum is smooth, while in the others the cortex is increased 

 by infolding and the formation of convolutions (gyri and sulci) 

 which reach their greatest complication in the anthropoid apes 

 and especially in man. A consequence of the increase in size of 

 the brain is the great development of the connecting nerve tracts, 

 which become more and more prominent as parts of the brain. 

 Thus the two halves of the cerebrum are connected by a large 

 transverse tract, the corpus callosum; two solid cords, the crura 

 cerebri, run back from the cerebrum to the other parts, while a 

 transverse commissure, the pons Varolii, passes below, connecting 



