THE REFLEX ARC 263 



several others, so that the nervous discharge which emanates 

 from it may be the efferent link in a very complex reflex arc. 

 This efferent discharge may descend to the appropriate motor 

 center, or it may ascend to enter a still higher association center, 

 all of whose afferent tracts come from similar lower centers and 

 therefore carry nervous impulses which represent a sort of physi- 

 ological resultant of the functional factors there interacting 

 [Herrick, 1913, pp. 223-24]. 



The cortex represents in fact a region into which 

 impulses originating in any receptor may under proper 

 conditions enter, but only after passing through one or 

 more correlation centers. Physiologically speaking, it 

 is the " central" nervous system par excellence in that 

 it is in a sense the meeting place of impulses from the 

 most various sources. 



The physiological conditions concerned in the origin 

 and development of the cortex must be conditions which 

 make possible or determine the localization of such a 

 truly central nervous region. Among these conditions, 

 those which have led to the development of the upward 

 paths discussed in the preceding section must have been 

 important factors. It has been suggested in chapter xi 

 that the primary directive factors in development of 

 the nervous system are electrical, arising in connection 

 with the physiological gradients and their modifications 

 and producing physiological effects through polarization, 

 and if that is true it is allowable to suggest that similar 

 electrical factors acting in the same way may be the 

 chief or the primary physiological factors concerned in 

 the ontogenetic localization and development of the 

 cortex. 



Gradients in electric potential are characteristic 

 features of the primary physiological gradients and 



