RESPONSE BY VARIATION OF ELECTRICAL RESISTIVITY 547 



resistivity, should bear so strong a resemblance to each other, 

 as is here seen to be the case, in the three records given side 

 by side (figs. 328, 329, 330). The excitatory effect may thus 

 be manifested by contraction, galvanometric negativity, or 

 diminution of resistance. We have already seen that the 

 electromotive is not a consequence of the mechanical re- 

 sponse, but is exhibited independently, when physical move- 

 ment is restrained. The response by resistivity variation 

 likewise, is, as we shall see, an independent expression of the 

 fundamental molecular change due to excitation. 



Having thus established the fact that true excitatory 

 response is exhibited by diminution of resistance, we have 

 next to ascertain whether this method of resistivity-variation 

 is capable of being employed in the study of excitatory 

 phenomena in general, with as great facility as those 

 mechanical and electro-motive methods with which we are 

 already familiar. In order to determine this question I 

 employed the same Wheatstone's bridge arrangement as 

 before. As it was important, for reasons previously given, 

 to use a non-electrical form of stimulus, I employed those 

 thermal shocks which we have already found to be so reli- 

 able. The thermal loop of platinum wire enclosed the 

 specimen as before, without being in contact with it. A 

 short-lived passage of heating-current, controlled by a metro- 

 nome, would now give rise to that sudden thermal variation 

 which we have seen to be effective in causing stimulation 

 It should be remembered that the rise of temperature, as 

 such, induces a responsive increase of resistance. But as, on 

 the other hand, the sudden thermal variation acts as a 

 stimulus, it should induce the excitatory response, by a 

 transient diminution of resistance. In the following experi- 

 ments, I employed the physiologically isotropic nerve of fern. 

 The resistance of this tissue, when balanced, was found to be 

 400,000 ohms. It should be stated here that this specimen 

 was in a very good tonic condition, and might be expected 

 therefore to give normal response. It was now subjected to 

 a series of stimuli of uniform intensity, at intervals of five 



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