CORPUS STRIATUM OF MAMMALS 123 



the adjustments which in reptiles and birds are con- 

 trolled from the striatum are in mammals taken up 

 by the cerebral cortex and there further amplified. 

 The mammalian striatum, accordingly, seems to be 

 a relict of a much more diversified organ, preserving 

 important but very sharply circumscribed functions. 



A resurvey of the entire question of the relations 

 of corpus striatum and cerebral cortex clearly shows 

 that the physiological factors chiefly responsible for 

 cortical differentiation in reptiles are to be sought in 

 the enlargement of the thalamic radiations and the 

 associated increase in the subcortical masses in the 

 lateral wall of the hemisphere, or otherwise expressed 

 in the larger participation of the hemisphere in the 

 exteroceptive reactions of the animal. The behavior 

 of the reptile in relation to the events taking place 

 in the world in which he lives is controlled from the 

 cerebral hemispheres in larger measure than in any 

 lower species. This control is partly effected in the 

 subcortical thickenings and partly in the cortex itself, 

 and the directions taken in the further differentiation 

 of the cerebral hemispheres in various species of 

 reptiles, birds, and mammals are determined in ac- 

 cordance with the relative parts played by these two 

 factors. 



It has already been pointed out that when differ- 

 entiated cortex first appears, in reptiles, it emerges 

 simultaneously in three functionally diverse fields, 

 on the medial, lateral, and dorsal aspects of the hemi- 



