226 BRAINS OF RATS AND MEN 



excitable cortex results in little obvious change in the 

 ordinary behavior or in acquired habits except those 

 involving rather complex kinesthetic adjustments. In 

 primates, this operation causes severe, though tem- 

 porary, impairment of all intentionally controlled be- 

 havior and in man this effect is much more severe and 

 enduring. 



This suggests that in the rat the behavior tested 

 involves a minimum of intentional control and that 

 the excitable cortex is largely concerned with dyna- 

 mogenic influence which is non-specific except in so 

 far as it is most closely linked with kinesthetic reac- 

 tions. In higher animals, on the other hand, inten- 

 tional control bulks larger in the total behavior and 

 the excitable cortex and pyramidal tracts are differ- 

 entiated as its chief (not exclusive) efferent mech- 

 anisms. 



In proportion as this type of control is amplified 

 the premotor cortex of the frontal lobe is elaborated. 

 This cortex seems to be concerned, in part at least, 

 with the organization of the results of cortical asso- 

 ciational processes in terms of the specific synergic 

 systems of muscles whose excitation is necessary for 

 the execution of the acts intended in purposeful move- 

 ment. That is, it is physiologically as well as ana- 

 tomically premotor. Its extent and complexity are 

 proportional to the range of variety and flexibility of 

 pattern of behavior in its motor aspects. It is sig- 

 nificant that in hoofed animals and carnivores, where 



