RESPONSE OF ANIMAL AND VEGETAL SKINS 



291 



due to, or modified by, the variation of contact resistance. 

 And if to avoid this the mechanical stimulus be applied, not 

 at the electrode, but at a neighbouring point, the results will 

 be quite different, according as the conductivity of the 

 intervening tissue is great or slight. In the former case 

 they will consist of the transmitted effect of true excitation ; 

 in the latter of the indirect effect, whose electrical sign is 

 the exact opposite. 



IP 



FIG. 176. Rotary Mechanical Stimulator 



Specimen of skin pinned on hinged platform which is pressed against 

 electrodes by elastic india-rubber. Electrodes rotated by cord, c c'. 

 s, antagonistic spring, made of elastic. Enlarged view of electrode 

 seen to the right. T, outer fixed brass tube ; T', inner rotating tube, 

 holding non-polarisable electrode, p, pumice-stone. 



A perfect method of direct mechanical stimulation has 

 been described in Chapter III., the stimulus being vibrational. 

 But for investigations on limp structures, such as skin, this 

 method is not practicable, and the modification which I am 

 now about to describe is necessary, in order to meet the 

 difficulties of the case. The apparatus consists of a hinged 

 platform, P (fig. 176), on which the specimen is securely 

 pinned. The two electrodes, E and E', rest with a definite 

 pressure on the two points A and B, whose excitatory re- 

 actions are to be studied. These electrodes have at their 





U 2 



