506 COMPARATIVE ELECTRO-PHYSIOLOGY 



seen to be a negative variation of the resting-current due 

 to injury. It is thus seen that while simultaneous excita- 

 tions of two normally excitable points E and E' are prevented 

 by balance from giving rise to any response, the excitatory 

 response becomes manifest when the balance is disturbed 

 by the injury of either point of galvanometric contact ; and 

 that, under these circumstances, the response is a negative 

 variation of the current of injury. This experiment is 

 important as giving a theoretical insight into the so-called 

 response by negative variation. It also shows how limited 

 is the applicability of the assumption that response is always 

 by negative variation. For, in the similar experiment, 

 previously described, under moderate injury, the response was 

 by positive variation of the resting-current. 



It is further seen from these experiments that the 

 enhancement of excitability, under the stimulation due to 

 moderate injury, could not be caused by the suggested 

 electrotonic effect. For the same anodic change induced 

 by injury at E causes, in the case of moderate injury, an 

 enhancement, and under greater injury a depression, of 

 excitability. It is thus clear that the modifying influence 

 is the effective intensity of stimulation. This fact, that 

 moderate stimulation enhances, and excessive stimulation 

 depresses excitability, will be further demonstrated in a future 

 chapter, by the independent method in which the effects of 

 electrotonus are completely eliminated. 



