THE FRONTAL LOBES 



229 



tentlonal control of the skeletal musculature. The 

 fact that after extensive destruction of this cortex, 

 with impairment of this control, the ape or man can 

 reacquire more or less of the control temporarily lost 

 is another and a very impressive illustration of the 

 plasticity of all cortical functions. This plasticity is 

 the most distinguishing feature of the cerebral cortex. 

 Stereotyped behavior patterns are largely provided 

 for elsewhere in the nervous system. The educability 

 of the cortex is its raison d'etre. And this implies that 

 its organization at any time can be readily modified, 

 that established patterns of internuncial association 

 can be replaced by new arrangements, and these in 

 turn by others, as external or internal conditions 

 change. 



Instead of assuming, as Lashley in the passage 

 quoted above seems to do, that the cortical control of 

 the finer adaptive movements is effected primarily 

 through the extrapyramidal fiber systems and that 

 the function of the pyramidal fibers is merely to acti- 

 vate or "prime" the lower motor neurons, thus pre- 

 sumably lowering their threshold for extrapyramidal 

 excitation, I would set out from a different analysis 

 of the physiological factors. Tonic or activating influ- 

 ences are exerted upon all behavior by both pyrami- 

 dal and extrapyramidal systems. The pyramidal sys- 

 tems, in higher primates, serve in addition as the final 

 common path for that sort of intentional control 

 which is the resultant of intracortical associations — 



