THE FRONTAL LOBES 223 



the intervening fields are small and poorly differ- 

 entiated. The outgoing fibers, as well as the incom- 

 ing sensory projection fibers, make direct connection 

 between their respective projection centers and sub- 

 cortical parts. There is no clearly differentiated "mo- 

 tor cortex'* through which integrated cortical func- 

 tions are discharged by a final common path, the 

 pyramidal tract. A part of the cortex is electrically 

 excitable, and from more or less of this field pyrami- 

 dal fibers descend to lower motor centers. This tract 

 is small and so far as now known it is concerned only 

 with certain types of kinesthetic activity, not with 

 other sorts of cortical function. 



In these forms cortical control of behavior is 

 largely effected through dynamogenic or tonic action 

 upon lower mechanisms of adjustment. This action 

 is chiefly extrapyramidal. Phasic action, or inten- 

 tional control, through the pyramidal system (cortico- 

 bulbar and cortico-spinal tracts) is at a minimum and 

 indeed is so slight that experiments hitherto reported 

 exhibit it scarcely at all. In man, tonic control of the 

 sort seen in the rat is in part localized in the corpus 

 striatum and so perhaps is capable of acting inde- 

 pendently of the cortex, and in part it is preserved as 

 the functions which are normally discharged mainly 

 through the extrapyramidal fibers. These leave all 

 parts of the cortex, and particularly the association 

 centers, as cortico-thalamic, cortico-rubral, cortico- 

 pontile, and other extensive systems of fibers. 



