228 BRAINS OF RATS AND MEN 



which can be pressed to activate some particular co- 

 ordinated system of lower motor neurons. Now, the 

 premotor cortex seems to be related to the excitable 

 areas in somewhat the same way that the mesence- 

 phalic centers of locomotion are related to the spinal 

 centers. That is, the premotor centers determine 

 which excitable areas will be activated and in what 

 sequence. But the premotor centers differ from the 

 mesencephalic centers in that, while the action of the 

 latter is rigidly predetermined by anatomically fixed 

 reflex connections, the action of the former fluctuates 

 from moment to moment as it is played upon by 

 nervous impulses from other parts of the cortex. 



This, I suppose, is the conventional notion of the 

 action of the motor cortex, and I see no reason for 

 abandoning it in view of the facts which Lashley 

 cites. It is clear, however, that in the past this organ- 

 ization has often been conceived too rigidly. 



We do not know the mechanism employed in cor- 

 tical motor control. It may well be in all cases a dif- 

 ferential dynamogenic or facilitating influence of 

 some sort, as Lashley suggests. Certain it is that "im- 

 pulses descending from the precentral gyrus do not 

 initiate the finer adaptive movements," for this is 

 the function of the total equilibrated action of the 

 cortex at the moment working. 



Yet the evidence seems adequate that in the 

 higher primates the precentral cortex in the normal 

 animal does lie in the final common pathway of in- 



