6l8 COMPARATIVE ELECTRO-PHYSIOLOGY 



evidenced by the progressive lessening of the positive 

 response, culminating in reversion to the normal negative. 

 This is equally true of nerve and metal. In the next class 

 of phenomena the modification of molecular condition is not 

 so great. It now exhibits itself merely as relative inertness, 

 and the responses, though positive, are feeble. Under 

 continued stimulation, they increase in the same direction as 

 in the last case that is to say, from less negative to more 

 negative^ this being the reverse of fatigue. This is evidenced 

 alike by the staircase effect and by the increase of response 

 after tetanisation, seen, not only in nerve, but also in 

 platinum and tin. The substance may next be in what we 

 call the normal condition. Successive uniform stimuli now 

 evoke uniform and equal negative responses that is to say, 

 there is no fatigue. But after intense or long-continued 

 stimulation, the substance is overstrained. The responses 

 now undergo a change from negative to less negative : 

 fatigue, that is to say, appears. Again, under very much 

 prolonged stimulation, the response may decline to zero, or 

 even undergo a reversal to positive, a phenomenon which we 

 shall find instanced in the reversed response of retina, under 

 the long continued stimulus of light. 



' We must, then, recognise that a substance may exist in 

 various molecular conditions, whether due to internal changes 

 or to the action of stimulus. The responses give us indica- 

 tions of these conditions. A complete cycle of molecular 

 modifications can be traced, from the abnormal positive to 

 the normal negative, and then again to positive, seen in 

 reversal under continuous stimulation.' * 



It is the molecular cycle here referred to, with the con- 

 comitant cyclic variation of response, that forms the subject 

 of the present chapter. I shall attempt to show that the 

 various anomalies in the response of living tissues, which 

 were referred to in an earlier passage, may be elucidated 



1 In the above quotation I have, in accordance with the convention which 

 I now uniformly follow, referred to normal response as negative, and abnormal 

 as positive. y. C r B< '. -.-'.. 



