8 COMPARATIVE ELECTRO-PHYSIOLOGY 



say, fatigue is found to depress the response of the inorganic 

 as of the organic (fig. 8). As in the case of animal tissues, 

 again, so also in that of metals, certain chemical substances 

 act as excitants, enhancing the response (fig. 9), others as 

 depressors and still a third class such as oxalic acid as 

 poisons, abolishing response altogether (fig. 10). 



By taking advantage of the last of these facts, we arrive 

 at a second means of obtaining response that is to say, the 

 method of relative depression. If both the contact points 



. Before | After 

 FIG. 9. Stimulating Action of Na 2 CO 3 on Electric Response of Platinum 



Records to the left exhibit response before, to the right after the 

 application of reagent. 



A and B be equally excited that is to say, subjected to diffuse 

 stimulation the responsive currents will be opposed, and 

 there will be no resultant galvanometric effect. This was 

 overcome, according to the method of block, by localising 

 excitation at one point, say, A ; we might, however, neutralise 

 altogether the counteracting excitatory effect at B by 

 abolishing the excitability of that point, as, say, by the 

 application of oxalic acid, A being left in its normal con- 

 dition. If now the wire be subjected to diffuse stimulation 



