RESPONSE OF ANIMAL AND VEGETAL SKINS 



from the normal negativity. That is to say, its response will 

 become absolutely positive, as represented by a above the zero 

 line. The response of the inner surface, B, may be normal and 

 strongly negative, as represented by e below the zero line. 

 When both these surfaces, then, are simultaneously excited, 

 the excitatory positive variation, or ' outgoing ' current at A, 

 will conspire with the ' ingoing ' current at B and the 

 resultant electro-motive difference will be B e + A rt , the direction 

 of the responsive current being thus from inner to outer. 

 But the same resultant up-response will also be induced, even 

 if the reaction of both surfaces be negative, provided only 

 that that of the outer, A a , be less negative than that of the 

 inner, B,, as explained by the diagram to the right of fig. 177. 

 The responsive current will then be represented as B e A a , 

 that is to say, as proceeding from the more negative B to 

 the less negative A. We have thus examined the two 

 extreme cases possible under the following formula, in which 

 the arrows show the direction of the responsive current, from 

 the more to the less induced negative : 



-- d-* c-*b-+ a. 



I shall next proceed to demonstrate the existence of 

 these two extreme types, taking vegetable skins as the 

 experimental specimen. It was supposed by Dr. Waller, as 

 will be recalled here, that owing to characteristic differences 

 between animal and vegetable protoplasm the response of 

 vegetable skin was opposite to that of animal skin : that is to 

 say, the former was ' ingoing ' and the latter ' outgoing.' That 

 this generalisation is not, however, justified, will be seen from 

 the experiments which I am about to describe, carried out 

 on the skin of grape. 



These results, it should here be pointed out, are not 

 dependent upon any one method of inquiry, for each 

 problem was subjected to attack and analysis by four 

 different modes of experiment. The first of these (i) was by 

 the Rotary Method of Mechanical Stimulation. This 

 method has the great advantage that by it the absolute 



