ORIGINS OF THE CEREBRAL CORTEX 77 



the thalamus, probably chiefly somesthetic (Huber 

 and Crosby, 1926). This field is enormously enlarged, 

 under the influence of more extensive thalamic con- 

 nections. 



The dorsal convexity contains simply organized 

 cortex. In the dorsomedial sector the cortical dif- 

 ferentiation which was foreshadowed in the frog is 

 here actually attained. This is hippocampal cortex, 

 in intimate nervous connection with the underlying 

 septum. The differentiation of this cortical sheet 

 seems to have taken place under the physiological 

 influence of subcortical olfactory-gustatory-visceral 

 reflex systems. 



Dorsolaterally, there is a pyriform sheet of cortex, 

 the precursor of the pyriform lobe of mammals. 

 This develops in relation with the underlying corpus 

 striatum and amygdala. Its fibrous connections in- 

 dicate that physiologically it resembles the hippo- 

 campus with the addition of strong exteroceptive 

 influences, probably chiefly somesthetic. 



Dorsally, there is a third cortical sheet, detached 

 from the other two, whose fibrous connections with 

 the corpus striatum indicate that its functions are 

 chiefly exteroceptive, with perhaps a motor center in 

 front (Johnston, 191 6). This we may call "general 

 cortex," or somatic cortex; it is the precursor of the 

 neopallium of mammals, as the other two cortical 

 sheets are of the archipallium. 



Each of these three cortical sheets is thus seen to 



