l6o COMPARATIVE ELECTRO- PHYSIOLOGY 



found that, in order to ensure death in the case for instance 

 of thermal section a prolonged application of the fatal 

 temperature is necessary. In ordinary cases of injury caused 

 by the application of heat, I find that we have merely exces- 

 sive stimulation of the point, with depression of excitability. 

 But after a long interval, excitability is more or less restored, 

 with the gradual passing away of the effect of injury. The 

 subsidence of the current of injury thus also denotes the 

 restoration of excitability to a greater or less extent. Hence 

 that differential action between the uninjured and injured 

 contacts, which determines the amplitude of the resultant 

 response, will become correspondingly diminished. And 

 when response has undergone diminution from this cause, 

 a fresh injury is found to renew its amplitude. This is due 

 to the reduction of excitability now freshly brought about at 

 one of the contacts. 



There are, however, two other additional factors which 

 may further contribute to the enhancement of response after 

 a recent injury. We have seen that, as a general rule, the 

 resultant response will be E A E Bj where E A means the 

 excitatory electrical change induced at A, and E B that induced 

 at B. It would therefore appear that this value will be at its 

 maximum when the excitability of B is totally abolished by 

 reason of injury, the resultant effect being due to the 

 unopposed electrical excitation at A. But we have seen 

 that when the true excitatory negative variation of a point 

 is abolished, it may nevertheless exhibit a positive electrical 

 variation, due to hydro-positive action. When this happens 

 to be the case, this positive effect at B, conspiring with the 

 true excitatory effect at A, may bring about a response larger 

 than we should have supposed to be maximum. 



Again, though over-stimulation of a point diminishes its 

 excitability, yet moderate stimulation often enhances it. 

 The effect of this, in enhancing resultant response, is well seen 

 in the case of conducting nerves. Thus, when the point B is 

 injured, the excitation caused by injury reaches A, and causes 

 moderate stimulation of that point. As an after-effect of this 



