142 BRAINS OF RATS AND MEN 



fusely. The outflow from the thalamus, downward in- 

 to lower motor centers or upward into the cerebral 

 hemispheres, will be determined as to its direction, 

 intensity, and other physiological characteristics by 

 the balance struck within the thalamus among the 

 nervous energies there released. 



This "equilibrium theory" of nervous action has 

 not as precise experimental confirmation as we would 

 like, but it seems to be consonant with the known 

 facts and to provide a serviceable viewpoint from 

 which to envisage the apparatus of compounding 

 simple reflexes and the establishment of new patterns 

 in associative memory and conditioned response. 



In terms of more general physiological conceptions 

 as Child (1924) and I (1924) have formulated them, 

 we may say that cerebral regions at the moment un- 

 der excitation are physiologically dominant over un- 

 excited parts and that the efficiency of this domi- 

 nance is dependent in part upon the intensity of the 

 excitation and in part upon the conductivity of the 

 intervening lines of transmission. Certain associa- 

 tional pathways are laid down at birth. Their con- 

 ductivity is good. These provide the mechanism of 

 the "natural" or innate action system. Other path- 

 ways of naturally higher resistance have their con- 

 ductivity improved during postnatal experience by 

 the method already suggested (p. 52) in our sche- 

 matic analysis of the conditioned reflex. Through 

 this secondary or conditioning process the "natural" 



