78 BRAINS OF RATS AND MEN 



be in most direct anatomical connection with its own 

 subcortical reflex center of characteristic physiological 

 composition. There is some localization of function 

 in the cortex from the inception of its differentiation, 

 a localization in space dependent upon a pre-existing 

 anatomical arrangement of the more ancient reflex 

 apparatus of the cerebral hemisphere out of which 

 the cerebral cortex has emerged. 



These three cortical sheets are related with one 

 another by intrinsic cortical association fibers in such 

 a way that one of them can probably perform no 

 function independently of the other two. This im- 

 plies that the interaction of these physiologically dif- 

 ferent cortical fields is probably an essential feature 

 of all cortical operations. 



This cerebral cortex cannot be activated directly 

 from the periphery, but only through the subcortical 

 reflex centers of the cerebral hemisphere and thala- 

 mus, which are more highly elaborated than in any 

 amphibians or fishes. Furthermore, there is no py- 

 ramidal tract, so that this cortex cannot act directly 

 upon the lower motor centers as the human cortex 

 does; it must exert its motor control through the 

 centers of co-ordination of the striatum, thalamus, 

 and midbrain. 



The reptilian cortex is not only simpler, but it is 

 much more intimately knit in with subcortical centers 

 than is the mammalian cortex. It is not capable of 

 so large a measure of independent or local autonomy. 



