124 BRAINS OF RATS AND MEN 



sphere wall, and that this specialization is correlated 

 with the increase in the thalamic radiations and with 

 enlargement and more sharp localization of functions 

 in the thalamus. That this increase in the participa- 

 tion of thalamic functions in the hemisphere is not 

 solely responsible for cortical differentiation is shown 

 by the fact that the first cortex does not appear ex- 

 clusively in the field which receives the largest inflow 

 of thalamic influences, that is, in the "general cortex'' 

 of the dorsal convexity, but it appears simultaneously 

 also in the olfactory areas medially and laterally of 

 this field, that is, in the hippocampal and pyriform 

 fields. This evidently means that the sine qua non 

 for cortical differentiation is an intimate connection 

 by means of association fibers within the hemisphere 

 between several fields, each of which is already a 

 tolerably complex center of correlation with different 

 functional connections from all of the others. And 

 the correlation centers in question must be partially 

 or wholly withdrawn from the main lines of direct 

 reflex connection, so that they are not dominated by 

 the innate stereotyped behavior patterns. 



The latter point is well illustrated by the history 

 of the further differentiation of these cortical fields 

 in mammals. The hippocampal and neopallial corti- 

 cal fields of mammals are detached in space and 

 isolated physiologically from the main thoroughfares 

 employed by the simpler reflexes of the brain stem, 

 and they undergo extreme differentiation of histo- 



