CURRENT OF INJURY AND NEGATIVE VARIATION 157 



takes place when the injury has not been excessive. If, 

 however this has been too great, the injured tissue will then 

 pass gradually into a condition of permanent death. But 

 the electrical change concomitant with death is one of 

 positivity, as I shall show in the next chapter. Thus the 

 subsidence of the galvanometric negativity of an injured point 

 may be brought about by either of two processes, which are 

 exactly opposite namely, recovery or death. 



Turning next to the subject of the Negative Variation 

 of an existing current of rest, as a reliable index to the state 

 of excitation, two different questions arise. First : why, in 

 order to obtain response to diffuse stimulation, is it necessary 

 previously to subject one of the contacts to injury? And 

 secondly : why is the responsive action-current opposite in 

 direction to the resting-current, thus constituting a negative 

 variation of it ? 



With reference to the first of these questions, we have 

 already seen that when two points, A and B, are simul- 

 taneously excited, the resultant electro-motive response is 

 equal to E A E B . If, then, the excitabilities of these two 

 points are the same, it is clear that the resultant response 

 will be zero. From this we can see that, in order to obtain 

 a resultant response, we must depress or abolish the excita- 

 bility of one of the two contacts. 



This inference may be verified by the employment of the 

 Method of Block and of longitudinal balance. Two equal 

 and opposite responses are first obtained at A and B. Then 

 one end, say B, is injured by thermal section. The specimen 

 being now replaced in the vibratory apparatus, it is found 

 that, whereas the A half gives strong response, the end B 

 gives none. Or the B end of the specimen may be injured 

 by a few drops of strong potash, the other end remaining 

 uninjured. The end A is then stimulated, and a strong 

 response is obtained. The end B is next stimulated, and 

 there is little or no response. The block between A and B is 

 now removed, and the specimen stimulated throughout its 

 length. Though the stimulus now acts on both contacts, 



