MECHANICAL RESPONSE OF NERVE 521 



the stimulus, and after a series of stimulations, the isolated 

 tissue, which was originally depressed, has its tonic condition 

 so much heightened, that the responses are enhanced to an 

 unprecedented magnitude. A specimen, in fact, which was 

 at first almost irresponsive, may generally be brought to 

 any state of exalted excitability desired, with concomitant 

 increase in amplitude of response, by merely subjecting it for 

 a certain length of time to the action of impinging stimulus. 



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FIG. 320. Photographic Record showing Abnormal Positive converted 

 into Negative Response after Tetanisation 



First series, abnormal positive ; second series, persistence of this positive 

 after very brief tetanisation ; third series, conversion to negative, after 

 a tetanisation of longer duration. 



I shall now describe in detail some of the principal 

 experiments. Selecting a specimen of frog's nerve, I took 

 a series of responses to electrical shocks, of three seconds' 

 duration, at intervals, in each case, of one minute. The 

 testing stimulus was kept always the same throughout the 

 experiment, except for certain intervening periods of tetani- 

 sation. The variations seen in the responses thus give 

 a visual demonstration of the variations in excitability. 

 The record of these is given in fig. 320. The responses in 



