662 COMPARATIVE ELECTRO-PHYSIOLOGY 



after-effect of previous stimulation, is seen by the shifting 

 upwards of the base-line. We may here refer back to fig. 3 1 

 where the responses of two different tissues to increasing 

 stimulus, show, in the one case complete recovery, and in 

 the other, a persistent after-effect. The base-line in the 

 former coincides with the line of absolute equilibrium ; and 

 increasing stimulus consistently shows an increasing response, 

 till, owing to molecular distortion reaching a maximum 

 value, a limit in the response was reached. In the second 

 of these records the persistent after-effect shifts the base- 

 line, which now becomes the line of modified equilibrium. 

 Owing to this fact, the relative maximum of variation 

 from the condition of modified equilibrium is reached much 

 earlier than would otherwise have happened. The extent 

 of individual response, after this, appears in fact to undergo 

 an actual diminution, though, measured from the line of 

 absolute equilibrium, this is not the case. In our sensation 

 we are unable to take cognisance of the absolute zero, 

 the changed condition of the nerve itself at any given 

 moment providing us ^with a relative zero, which is our 

 only standard of comparison, and our whole perception of 

 intensity depends upon induced variations from this chang- 

 ing zero. 



The record which I have given in figs. 400 and 401, show- 

 ing the mechanical responses in the nerves of gecko and frog, 

 under increasing stimulus, not only explains the quantitative 

 change in the sensation, but also that change of quality or sign, 

 under appropriate conditions, which had not hitherto found any 

 explanation. We see in the curve of response, as has been 

 said, that with feeble stimulus response is positive^ and that 

 this positive response attains a climax, after which it diminishes 

 in amplitude, and then changes sign and passes over into 

 negative under increasing stimulation. In the corresponding 

 response by sensation, we find similarly, not only a difference 

 of quality or sign, but also a transformation from one to the 

 other, under increasing stimulation, from a climax where the 

 sensation attains a maximum of the pleasurable tone. After 



