PHYSICAL BASIS OF SENSATION 66 1 



being equal, the intermittence falls below the rate of thirteen 

 per minute. An intensity of this intermittent radiation 

 stimuli, at first not painful, and yielding sensations strikingly 

 discrete, soon becomes dolorific, and then the sensations 

 remain discrete no longer, but are fused more or less to- 

 gether.' l 



This anomaly of gradual heightening of sensation, and 

 later, fusion of effects, appears at first sight inexplicable. I 

 shall give a satisfactory explanation of the latter point in the 

 next chapter ; but as regards the former of its two elements 

 namely, the heightening of sensation under uniform intensity 

 of stimulus we have seen that, owing to the after-effect of 

 stimulus, the condition of responding substances in general, 

 and nerve in particular, is gradually transformed, from a 

 point below the transition B to one above it. But, owing to 

 this transformation, the character of the response is changed 

 (cf. fig. 382). If the original response be positive, it will be 

 converted later into negative. If it be moderately negative, 

 it will be converted into more intense negative. 



It will also be seen that the intensity of response is not 

 solely determined by the intensity of stimulus, but is modified 

 by the existing condition of the nerve. And this latter under- 

 goes a change by the action of stimulus itself. Thus there 

 are certain times of the day when, owing to sluggishness 

 of the tissues, our power of perception is dull. But acuity 

 of perception becomes enhanced with each successive stimulus 

 responded to. This is also true, more or less, of each indi- 

 vidual undertaking. These facts are easily understood from 

 a consideration of the different responses which are charac- 

 teristic of the ascent of the molecular curve above the point 

 of transition. 



We have also to consider the fact that this progressive 

 molecular modification, under certain circumstances, imposes 

 the anticipation of the responsive maximum, with increasing 

 stimulus, or even, in other cases, the induction, of an actual 

 decline. The molecular variation, in consequence of the 



1 Text-book o f Physiology > edited by Schafer (1900), vol. ii. p. 998. 



