DISSOCIATION OF COMPLEX SENSATION 667 



citability of the stimulated point, and the conductivity of the 

 transmitting tissue. We shall, therefore, investigate separ- 

 ately the effects of various agents in the modification of 

 receptive excitability and of conductivity respectively. 



We saw, again, in the last chapter, that as the intensity of 

 stimulation is continuously increased, the response gradually 

 passes from the positive, through the point of transition, into 

 the negative. This happens, as we have seen, immediately 

 above the point B. This region, therefore, may be referred 

 to as critical and indifferent, and must be regarded as having 

 the peculiarity that if, by any means, the molecular curve be 

 raised above it, the corresponding sensation tends to become 

 actually painful, or if carried below, becomes pleasurable. 



It follows that, in this critical indifferent region, any 

 condition which tends to exalt the excitability of the tissue, 

 and thus the negativity of the response, will also accentuate 

 the negative tone of sensation, its mixed character thus 

 passing into the distinctly painful. Those conditions, on the 

 other hand, which lower excitability, and therefore responsive 

 negativity, will also act by detracting from the painful 

 element in the indifferent sensation in the critical region, and 

 rendering it to a greater or less extent positive, soothing, or 

 pleasurable. In this way, by depressing or raising the 

 excitability at will, a stimulus which had already caused a 

 painful sensation might be made not unpleasant, and an 

 indifferent or positive sensation could be made negative or 

 painful. We shall presently see how, by variations due to 

 the presence of internal or external modifying factors, the 

 positive tone of sensation is actually rendered negative, and 

 vice versa, under such transpositions. 



Now we have at our disposal various well-known agencies 

 by which the excitability of the tissue can be raised or 

 lowered. In order to lower excitability, certain anaesthetic 

 agents may be employed, and their effect in the modification 

 of sensation-tones will presently be described. The ideally 

 perfect means, however, not only for the exaltation or 

 depression of excitability at will, but also for the rapid 



