230 BRAINS OF RATS AND MEN 



for the phasic action of the cortex as an arbiter of 

 conduct. 



It is normally the extrapyramidal fibers that exert 

 the chief non-specific tonic or activating effect, where- 

 as the action of the pyramidal fibers is specific and 

 phasic. Phylogenetically, the specific phasic system 

 has developed from the non-specific tonic system. 

 Now, upon loss of the final common path commonly 

 employed by the phasic system — the pyramidal 

 fibers — the plasticity of cortical organization is such 

 as to permit, during re-education, of a reorganization 

 of cortical connections which in a way recapitulates 

 the ancestral history. That is, more or less of the non- 

 specific extrapyramidal fibers secondarily take up the 

 functions of the lost pyramidal fibers, and the phasic 

 cortical activities find a new outlet. An analogous 

 sort of functional readjustment takes place after 

 grafting of peripheral nerves with anastomosis in new 

 patterns, so that we know that the process suggested 

 is neurologically possible. 



It is concluded, accordingly, that the "motor 

 cortex" primitively was developed within the field of 

 kinesthetic-motor cortical control as the efferent path 

 for its own type of cortical activation only, just as 

 within the occipital pole there is visual motor cortex 

 within the general visual field. With further differen- 

 tiation of premotor associational mechanisms, chiefly 

 in the frontal region, the precentral motor cortex be- 

 came progressively more important as a link in the 



