IV. VEErEBRATA : MAMMALIA. 



623 



and fishes, the cerebeUum (/F)is differentiated into a median 

 vermis and lateral cerebellar hemispheres. In the cerebrum the 

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

 wards over the olfactory lobes, which consecinentlj^ 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 tlie 

 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 (tig. (i-tO, A) the olfactor}' lobes arc 



Fig. 640. — A, brain of rabbit (after Gegenbaur); B. of fish otter. C, of pavian 

 nionliey 'after Leuret ami Gratiolet). I, cereijruin; III. optic lobes; JF, cerebel- 

 lum; T^ medulla oblony:ata ; ^j, olfactory loljes. 



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

 carnivores (fig. 040, B), and ungulates the olfactory lobes are 

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

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

 cerebral hemispihores 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 conseqitenco of the increase in size of 

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

 which become more and more j^rominent 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 tlie cerebrum to the other parts, while a 

 transverse commissure, the pjons Varolii, passes below, connecting 



