VARIOUS TYPES OF RESPONSE 



101 



from molecular considerations will be seen from the fact 

 that effects exactly similar are met with in the response of 

 inorganic matter (cf. figs. 258 and 383.) 



In the case of responses exhibiting fatigue from over- 

 strain, we have a diminution of normal response, which may 

 ultimately culminate in reversal. We may imagine a spiral 

 spring, undergoing increasing compression from a gradually 

 augmenting force. The responsive compression will at first 

 be considerable. But this will soon reach a limit, beyond 

 which added force will seem to have but little power to induce 

 further responsive distortion. In a somewhat similar way, 

 we may visualise the condition of the responding molecule at 

 the stage D or E. Here, molecular distortion has almost 

 reached its limit. It follows that added stimulus can induce 



FIG. 72. Fatigue in the Contractile Response of India-rubber 

 Note the periodic alternation and the reversal at the end. 



little further distortion. But the maximally distorted mole- 

 cule has now a great tendency to revert to the position of 

 equilibrium, and the shock of stimulus, instead of inducing 

 excitatory action, induces the reverse. That this is to be ex- 

 plained by molecular rather than chemical considerations, 

 is seen in the following record (fig. 72) of the contractile 

 response of india-rubber to thermal stimulation. This 

 represents the last part of a long series of responses, whose 

 amplitude was already undergoing a progressive decline. 

 Further symptoms of growing fatigue are seen in the periodic 

 alternations of amplitude, and in the final reversal of response 

 to one of expansion. I shall later give another record in 

 which the normal negative response is seen' reversed to 

 positive through an intermediate diphasic. 



