104 THE SIGNS OF LIFE [LECT. 



applied to subcutaneous tissue will serve us as an indifferent 

 electrode, under which little or no local response to excita- 

 tion need be expected. But we must take care that the 

 wound does not involve injured or exposed muscle, we need 

 only transfer our " indifferent " electrode to an injured muscle 

 (or to an injured nerve) to be convinced that the latter is 

 strongly electromotive in the usual (zincative) sense. The 

 hint may, perhaps, prove of some service to us ; we may one 

 day want to make use of an indifferent electrode for the 

 further investigation of the electrical reactions of undisturbed 

 skin or mucous membrane. Reflect for an instant we must 

 always get a resultant of two factors, if our circuit includes 

 two active surfaces, whether both be of skin, or both mucous 

 or one skin and one mucous. So that to get at the reaction 

 of one spot of skin in situ or of mucosa in situ, it will greatly 

 simplify our task if the subcutaneous tissue may be treated 

 as electrically indifferent and non-responsive. 



63. Review. We have gone over the whole ground, and 

 rather more rapidly than I expected. These three experi- 

 ments exhibiting, first, the ingoing current aroused by in- 

 direct excitation ; second, the ingoing current aroused by not 

 too strong direct excitation of a fresh pad ; third, the outgoing 

 current aroused by strong direct excitation of a twenty-four 

 hour pad very fairly summarise all I have learned about these 

 currents during the last few months. Some few details may, 

 indeed, be added the lost time of indirect excitation for instance, 

 and the rapid exhaustion of the indirect effect by repeated 

 stimuli ; but these will be best shown to you by projecting on 

 the screen the actual records of these phenomena. And I may 

 mention that in one experiment directed to the point, it was 

 found that no effect was produced by excitation post-mortem 

 of the sciatic nerve of a previously atropinised cat, and that in 

 another experiment the same state was found on a cat killed 

 by chloroform * ; in both these experiments the effects of direct 

 excitation were found to be normal. One other point in both 



* In other cats, chloroformed and killed, the indirect effects have always 

 been normal j the instance quoted in the text was exceptional. 



