6l2 COMPARATIVE ELECTRO-PHYSIOLOGY 



(fig- 37 that, owing to imperfect conductivity of the inter- 

 vening tract, but little excitation reaches it. Excitation at S, 

 however, distorts the molecules in its immediate neighbour- 

 hood, in a certain direction, incipiently distorting others at a 

 little greater distance in the same favourable way. A second 

 stimulus is therefore transmitted a little further, bringing 

 about the same predisposition still further on. Thus an im- 

 proved conducting-path is made, in a substance formerly but 

 an indifferent conductor, by the action of the stimulus itself. 

 In this way transmitted excitation, at first relatively ineffec- 

 tive, becomes increasingly effective (fig. 377). It is very 

 interesting to note that I have obtained an effect exactly 



parallel in the case of nervous tissues. 

 For example, when attempting to 

 obtain the transmitted effect of ex- 

 citation by mechanical response, in 

 a vegetable or animal nerve in de- 

 pressed tonic condition, the first series 

 FIG. 377. Gradual Enhance- of tetanising shocks would induce no 

 ment of Conductivity by responsei It would sometimes be 



the Action of Stimulus v 



only after long repetition that con- 

 ductivity would be gradually restored, as seen in the initiation 

 and subsequent enhancement of responses given at the distant 

 responding-point. 



It may be that few phenomena connected with the 

 response of living tissues, bring home to us, so effectively 

 as an experiment on a nerve- and-muscle preparation, the 

 sense of the specific and mysterious character of the 

 responsive manifestations of the living. The nerve, at its 

 central termination, is locally excited by electric shocks, 

 and some obscure impulse then passes through the long 

 conducting tract to the muscle at the other end. Arriving 

 there, this invisible nervous impulse initiates a new series 

 of events, which find expression in visible motile indications 

 The work performed at the responding end may be out of 

 all proportion to the strength of the stimulus imparted at the 

 centre. It is as if the nervous impulse tapped a relay, and 



