704 COMPARATIVE ELECTRO-PHYSIOLOGY 



outer. Here the resultant current is brought about by 

 the difference between the stronger responsive negativity of 

 the inner, and the feebler responsive negativity of the outer 



surface, which may be represented as jj. With certain 



specimens of tomato skin, however, the modification of the 

 outer surface is so great that its individual response is 

 reversed to positive, that of the inner being the normal 

 strong negative. The resultant response, then, is still from 



inner to outer, but equals the summated effect of the two I, 

 From this we pass to the response of grape-skin, which 

 resembles the latter of these two cases. The response of 

 the skin of frog is also of this type, and it may be said 

 of skins in general that their response is from the more 

 excitable inner surface to the less excitable outer. This 

 conclusion has been verified by experiments on various 

 skins, both vegetable and animal. Among the latter of these 

 may be mentioned the skin of the neck of tortoise, and that 

 from various parts of the body of gecko. 



When the skin is isolated with very great care, so as to 

 reduce to a minimum the excitatory effect of preparation, it 

 is found that the natural current of rest is from the less 

 excitable outer to the more excitable inner surface, the 

 excitatory current being in the opposite direction. Owing to 

 the excitatory effect of preparation, the current of rest of the 

 skin of tortoise was found reversed. The responsive current, 

 however, was found to flow from inner to outer, thus proving 

 that the inner surface was the more excitable. In illustration 

 of the great practicability of the methods employed, I may 

 refer to the photographic records obtained of the response of 

 the skin of the intact human forefinger (fig. 180). 



In describing the differential excitability of the hollow 

 peduncle of Uriclis lily, it was shown that protected surfaces 

 are, as a rule, more excitable than those which are exposed, 

 and have thus undergone a greater degree of modification. 

 On taking the plagiotropic stem of Cucurbita, the lower 

 surface of which is protected from light, it is found that, 



