v.] BOTH POLES ARE EFFECTIVE 93 



been alluded to above, and now demand our somewhat closer 

 scrutiny. Is a blaze-current exclusively post-anodic or post- 

 kathodic, or both, and if both, which is the more effective pole ? 



The question can be experimentally tested in three ways 

 the exciting current can be sent through the skin by two 

 electrodes in contact with its two surfaces (Fig. 26), or through 

 electrodes both of which are applied to the external surface, or 

 with three electrodes A B C on the external surface (Fig. 33). 



By the first of these three plans we have already seen that 

 with both directions of excitation an outgoing response is pro- 

 voked ; we have seen farther ( 43) that both these responses 

 depend on an electro-negative state at or close to the external 

 surface. Both poles produce this change, and the fact (illustrated 

 by Fig. 30) that the antidrome is sometimes greater than the 

 homodrome response, signifies at first sight that the anode is 

 more efficacious than the kathode. But this conclusion is not 

 confirmed by our next two experiments, and the inequality of 

 response, which, indeed, I find on reference to my notes, is by 

 no means invariable, may have been due to other causes, e.g., 

 to ordinary polarisation currents against homodrome and with 

 the antidrome response, the true physiological response being 

 greater in the homodrome sense. 



The second plan of experiment, by two external electrodes, 

 gives a result that can hardly consist with superiority of a post- 

 anodic effect. The response in this case is always homodrome, 

 and since it is the algebraic sum of two opposed outgoing currents 

 at the two poles, this signifies that the kathode has been more 

 efficacious than the anode. A homodrome post-anodic response 

 would have been ingoing, but as we have seen, and shall see 

 again, it is the post-anodic response which is antidrome and 

 outgoing. This experiment exhibiting homodrome response 

 with two external electrodes is easily repeated ; the result is 

 very constant and contrasts sharply with that obtained when 

 the two electrodes are applied to the internal surface ; this 

 surface is ineffective, and gives only small antidrome deflections 

 attributable to ordinary polarisation, whereas the external sur- 

 face gave large homodrome deflections of which the physiological 



