PHYSIOLOGICAL PARADOXES. 



The nerve endings, which are sensitive to 

 light, are liable, on excessive stimulation, not 

 only to fatigue, but to reaction in the form of 

 automatic negative stimulation. For a very 

 rough analogy to the result, which does not 

 pretend, however, to explain the mechanism, 

 we may consider a steel spring. If we disturb 

 this from its neutral position by bending it to 

 one side, the further we do this the more diffi- 

 cult it becomes to do it further still, and the 

 more strongly the spring tends to bend back past 

 the neutral point in the opposite direction. 



If our visual nerves are stimulated by all 

 the rays of light which go to make orange, they 

 undergo fatigue for that colour stimulus orange 

 is less able to produce a vivid sensation. 



Not only so, but when that stimulus is with- 

 drawn, the result of the first stimulation is a 

 reacting or negative stimulation, producing a 

 sensation of what may be called the opposite 

 kind of light that is, the light which would 

 result from all the rays of white which were not 

 present in the original stimulus. 



It seems a pity that the negative or re- 

 acting effect, producing an opposite sensation, 

 is not found in more of our nerves. When the 

 nerve of a tooth has, by contact with food, been 

 stimulated to agony, the immediate removal 

 of the stimulus is by no means followed by the 

 opposite sensation of pleasure. On the contrary, 

 the agony often grows keener with time. 



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