REFLEX AND CONDITIONED ACTION 57 



way between the auditory and the saliva tory paths; 

 but by reason of the spread of these auditory excita- 

 tions throughout the correlation center the entire 

 center is partially activated. This activation is ade- 

 quate at first to produce a reaction only through the 

 natural pathway leading down to the spinal cord for 

 turning the head. If the ascending gustatory path 

 is simultaneously acting upon the correlation center 

 and thence down by its own path to the salivatory 

 nucleus, this latter discharge is reinforced or facili- 

 tated by the diffuse excitation already acting from 

 the auditory path. The threshold for the salivatory 

 reflex is thereby lowered because the entire center is 

 partially activated and "primed" in advance. For 

 illustrations of such reinforcement see Yerkes (1905); 

 for the mechanism involved see Herrick (1913, 1917). 

 It is well known physiologically that a nervous 

 circuit which is in action is prepotent over resting 

 circuits, and when a response is in process intercurrent 

 stimuli tend to affect the acting circuit rather than to 

 activate resting systems of neurons which would 

 otherwise be excited. This is usually ascribed to the 

 greater permeability of the active systems, whose 

 internal inertia has already been overcome so that 

 they tend to capture or draw x)ff available nervous 

 energies from surrounding parts. In accordance with 

 this principle the diffuse or non-specific activation of 

 the correlation center by the auditory stimulus finds 

 an outlet already open through the descending 



