236 BRAINS OF RATS AND MEN 



termine the size, number, and arrangement of the 

 association centers in other than premotor territory. 

 In the archipallium the organization is different. 



As we pass from the lowest mammals toward the 

 higher the projection centers become more sharply 

 defined, structurally more dissimilar from one an- 

 other, and physiologically more evidently indispen- 

 sable links in the chains of higher associational pro- 

 cesses. The intervening cortex, the associational cor- 

 tex, is enlarged and differentiated. These cortical 

 areas have been mapped anatomically and investi- 

 gated physiologically in representatives of most 

 mammalian groups. We shall not attempt to sum- 

 marize these studies, which would require a large 

 volume. The process of cortical differentiation cul- 

 minates in the human brain, where upward of fifty 

 cortical areas can be distinguished by differences in 

 anatomical structure. 



Now, apparently in the rat the cortex has reached 

 a stage of elaboration at which cortical facilitation of 

 lower activities is exerted more or less differentially, 

 but the intrinsic functions of cortical association are 

 poorly represented as compared with higher mam- 

 mals. These latter appear to grow pari passu with 

 further elaboration of the associational tissue inter- 

 polated between the projection centers. 



Habit formation through repetition and learning 

 by the method of trial-and-error, as we have seen, 

 are not primarily cortical functions, though they may 



