DISSOCIATION OF COMPLEX SENSATION 675 



the positive or sensation of contact, followed by that of 

 pain. With still greater depression of conductivity, the 

 normally painful sensation, by the blocking of its negative 

 element, will be turned into one of positive tone. 



We shall now proceed to verify these theoretical inferences. 

 The fact that there are actually two distinct nervous impulses, 

 and that of these the positive travels faster than the negative, 

 is demonstrated by the well-known experiment in which a 

 smart tap is applied on the ball of the foot, with the result 

 that we perceive, first, the sensation of touch, and then, at an 

 appreciable interval afterwards, that of pain. The possibility 

 of this demonstration in a normal nerve is due to the length 

 of the conducting nerve here concerned. 



In other cases the same dissociation is seen to be effected 

 under varying degrees of loss of conductivity, in different 

 forms of nervous paralysis. The considerations which I have 

 advanced, will, however, I believe, offer a satisfactory explana- 

 tion of those curious instances of ' dissociation ' and ' delayed 

 pain ' which are known to pathology. In such cases, a prick 

 with a pin, for instance, is first perceived as mere contact, 

 and then, after an appreciable interval, as the sense of pain. 

 Other cases are known in which, the loss of conductivity 

 being very great, the patient could handle burning coal with- 

 out pain, the resultant perception being entirely positive. 



From the diverse phenomena described in this and the 

 previous chapters, it is clear that a sensation of positive tone 

 is associated with that particular responsive change in the 

 nerve, which is expressed as expansion and galvanometric 

 positivity. We have also seen that such effects are brought 

 about by a feeble intensity of stimulus, which acts to increase 

 the internal energy without inducing the true excitatory 

 negative effect. Of anything which thus increases the 

 internal energy, it may be said in general that it will induce 

 positive mechanical and electrical expressions with concomi- 

 tant positive sensation. With stronger stimulus, as we have 

 seen, the negative mechanical and electrical responses are 

 induced. But such negative responses contain, as we have 



x x 2 



