8o BRAINS OF RATS AND MEN 



In reviewing the organization of the reptilian 

 cerebral hemisphere two features are noteworthy: 

 First, the cerebral cortex is differentiated in a dorsal 

 field lifted up above the lower reflex centers of the 

 hemisphere and the great lines of fore-and-aft trans- 

 mission of the brain stem. It is detached in space from 

 the more primitive reflex arcs. It is shunted off above 

 the great lines of through traffic that serve the 

 fundamental innate reflex and instinctive activities. 

 Second, three separate cortical sheets appear simul- 

 taneously overlying three more ancient subcortical 

 correlation centers. These lower centers differ among 

 themselves in the reflex functions which they per- 

 form, and in reptiles and birds they are greatly en- 

 larged. Cortex is never differentiated in a single 

 physiologically homogeneous field, but always simul- 

 taneously in several dissimilar fields. No one of these 

 cortical sheets can be thought of as the exclusive 

 organ of any function, but each is probably essential 

 to the normal working of both of the others if the 

 highest efficiency is to be attained. 



These observations suggest two additional things: 

 first, that the cortical functions are something added 

 to the innate reflex and instinctive endowments, not 

 merely more of the same kind of reflexes; and, second, 

 that the most elementary cortical functions involve 

 the association within the cortex itself of diverse 

 physiological complexes in patterns which are not 

 provided for in the more primitive subcortical centers. 



