7 6 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



mammals and of man become less and less convoluted, 

 until they become entirely smooth. The same is true 

 as we go back in the evolution series. Extinct mam- 

 mals have less and less convoluted brains as we go back 

 in time. All the earliest mammals had smooth brains. 



3. Relative Size of Cerebrum. — The cerebrum is 

 confessedly the highest part of the brain. The relative 

 size of this is the best of all tests of position in the scale 

 of organization. Observe, then, that there are four 

 lobes used in this comparison, viz., the cerebellum, the 

 optic lobes, the cerebrum, and the olfactory lobes ; for 

 this last is an important lobe in all lower vertebrates. 



Now in man, as already seen, the cerebrum, growing 

 out from the thalamus, spreads forward, covering en- 

 tirely the olfactory lobes, and backward, covering first 



Fig. 41.— Mammal brain. A, top view ; B, side view of the brain of a cat. 



the optic lobes, then the cerebellum, until looking down 

 upon the brain we see nothing else. It covers and 

 dominates all. In monkeys, by a less backward prolon- 

 gation, the cerebellum begins to peep out behind In 

 the average mammals, such as the lion or the dog, the 

 olfactory lobes are exposed in front, and nearly the 

 whole of the cerebellum isuncovered behind (Fig. 41). 

 Still lower among mammals the cerebellum is wholly 

 uncovered and the optic lobes begin to appear, and all 

 the four lobes are seen in a series (Fig. 42). The brains 

 of extinct mammals are all of this low type. 



