A LEAP IN THE DARK 32 



In all these cases the response depends upon 

 the ' excitability ' of the responding tissue ; 

 and when we examine the matter further we 

 find that the excitability is dependent on a very 

 large number of conditions, most of which are 

 presumably still unknown. Moreover, the 

 excitability varies in response to very minute 

 changes in the environment, just as is the case 

 with the original response to the stimulus. 

 The presence or absence of some blood- 

 constituent, normally present perhaps in only 

 the minutest proportions, may profoundly 

 affect the excitability of any tissue : or its 

 response may be modified or * inhibited ' by 

 excitation of other parts. It is true that 

 physiologists can obtain a constant response to 

 a given stimulus ; but this is only the case 

 if the conditions are 'normal/ In living and 

 intact organisms nature provides us with nor 

 mal conditions. The more carefully the matter 

 is investigated, however, the more clearly does 

 it appear that 'normal conditions' designate 

 something of vast and unknown complexity. 

 The fact that such conditions should be realised 

 again and again in different individual organ 

 isms, and in the same organism at different 



