DYNAMOGENIC INFLUENCE OF LIGHT 373 



the performance of the task is expended in the act of discriminat- 

 ing among the " suits" by the senses of touch, the inference seems 

 justified that tactile sensitivity is enhanced by uniform stimu- 

 lation of the retina by light, even though the visual and tactile 

 impressions cannot be referred to the same object, and vision 

 cannot serve even as a means of orientation. 



Of the several hypothetical explanations which are possible, I 

 shall mention only the one which seems to me to be the most 

 useful. 



Let us first admit the postulate that a differential response of 

 some system of muscles is as essential a part of the process of 

 perception, or of discrimination, as is the excitation of a nerve- 

 center, or even the stimulation of a receptor. Then, any condi- 

 tion which tends to increase the tonicity of the muscles, or in 

 other words, renders them more ready to contract when stim- 

 ulated, will tend to lower the threshold and shorten the time re- 

 quired for perception and discrimination. 



Such an increase in tonicity is actually brought about by the 

 action of adrenalin, and probably by other products of internal 

 secretion of whose manufacture little is now definitely known. 



A similar result would ensue from the action of a similar product 

 which might tend to sensitize the synapses. 



Conversely, one might attribute the diminished sensitivity in 

 darkness to the action of some endocrinal product which tends to 

 diminish the tonicity of the muscles, or to retard the activity of 

 the synapses, and which is produced in greater quantities in 

 darkness than in light. 



Either of these hypotheses would explain in terms of strictly 

 physiological concepts the differential effects which were ob- 

 tained, as the poorer performance in darkness appears to be due 

 specifically to the absence of light and not to elimination of vision 

 of objects. 



The production of either of these hypothetical substances may 

 be regarded as a conditioned reflex, occasioned by long habitua- 

 tion to activity in the light and a minimum of activity, tending 

 toward somnolence, in the dark. That this habituation is capa- 

 ble of modification by training is suggested by the record of 



