174 COMPARATIVE ELECTRO-PHYSIOLOGY 



Hence, having one contact fixed on a living area and 

 the other on (i) the dying, (2) the intermediate, and (3) 

 the dead tissue, we shall obtain three different types of 

 what is known as the ' injury-current.' In the first of these 

 the second contact will be negative, a condition which has 

 hitherto been assumed to be the sole characteristic of the 

 current of injury. But there are two other cases to be 

 considered. Of these, when the second contact is made at 

 a point intermediate between the dying and dead tissues, 

 we shall find it to be iso-electric with the first, or living 

 contact. And thirdly, when the second contact is on a 

 dead area, the latter will be positive to the first, or living 

 contact. We thus find three cases of the current of injury 

 the first being negative, the second zero, and the third 

 positive. 



Taking the first of these that in which the injured 

 contact is negative the action-current, in response to 

 stimulus, will bring about a negative variation of the so- 

 called current of injury. In the second, the result will be 

 indeterminate, since the injury-current is zero. In the third, 

 the response will be by a positive variation of the current of 

 injury. 



I give below three photographic records in illustration of 

 these three cases, obtained with vegetable nerve. I may state 

 here that I have often observed results precisely similar in 

 the case of frog's nerve also. In the first record, in fig. 116, 

 the thermal injury was moderate. The injured point was 

 thus negative, and the current of injury is represented here 

 by an up-line. The responses are seen to be by negative 

 variation. In the second record the injury was greater, and 

 the injured point was almost neutral ; that is to say, on 

 making contact there was a slight up-twitch, which subsided 

 to zero. There is here, then, no current of injury. The 

 subsequent responses are, however, down, the action-current 

 being away from the living contact. In the third record 

 the injury was so great as completely to kill the injured 

 point, which thus became positive to the living. The 



