70 BRAINS OF RATS AND MEN 



the cerebral hemisphere (corpus striatum complex) 

 in these animals are enormously developed. 



3. The mammalian type. — In even the lowest 

 mammals the cortex is suddenly expanded in amount 

 and elaborated in internal texture. And the process 

 of further elaboration goes on progressively through- 

 out this group. 



4. The human type, — Man is a mammal and his 

 brain in all respects but one is essentially similar to 

 that of an ape. But the association centers of his 

 cortex are magnified and in internal texture compli- 

 cated in measure commensurate with his enlarged 

 and glorified mental powers. 



These four types or stages are discontinuous. 

 That is, there are no existing intermediate forms. 

 This means, of course, that the transitional species 

 are now extinct; and these missing links have left 

 no record of the internal structure of their brains. 



Throughout this series of progressively more 

 comphcated cortical patterns no fundamentally new 

 structural features are added. The most highly elabo- 

 rated cortex is made up of neurons of substantially 

 the same sort as those of the rest of the brain. Ana- 

 tomically, this progressive evolution appears to be 

 wholly a matter of increase in number of neurons and 

 gradual change in the structural arrangements by 

 which they are physiologically interconnected. We 

 shall next examine briefly the arrangement of the 

 cerebral cortex in representatives of these four types. 



