MECHANICAL RESPONSE OF NERVE 523 



of frog's nerve. In this, also, the first series of responses 

 was purely positive. But the record shown here begins at 

 the point where, in consequence of previous tetanisation, 

 response has become diphasic. Here it will be noticed that 

 the true excitatory effect of contraction is undergoing a con- 

 tinuous increase, while the abnormal positive is decreasing. 

 The excitatory response, indeed, becomes so great as to be 

 incapable of record within the plate. 



I have already shown how similar in every respec 

 are the responsive characteristics of the vegetal nerve to 

 those of the animal. This fact finds an interesting illus- 

 tration in the various phases of its mechanical response. 

 That is to say, plant nerve in a sub-tonic condition gives 

 positive, passing into diphasic and normal negative response, 

 under tetanisation. On arriving at this second stage of 

 diphasic response, successive responses undergo enhance- 

 ment in a manner precisely the same as holds good in the 

 corresponding cases with frog's nerve. This is sufficiently 

 illustrated in the two records given side by side in 

 fig. 321, the first of which, as already said, is of frog's nerve, 

 and the second, of nerve of fern. We see here again, as 

 already in numerous cases before, how" the responsive pecu- 

 liarities and their modifications in the one are in every 

 respect paralleled by those of the other. The only differ- 

 ence between them lies in the degree of their excitability, 

 that is to say, two stimuli of equal intensity will in general 

 induce a more intense excitatory effect in the nerve of frog 

 than in that of fern ; or in order to obtain from both an 

 equal intensity of response, we must, in the case of fern, 

 employ a stronger stimulus. We have seen that in con- 

 sequence of the absorption of stimulus, not only does the 

 abnormal positive phase disappear, giving place to the 

 normal negative, but the subsequent negative responses 

 themselves also show an enhancement in a staircase manner. 

 I give here (fig. 322) another record showing the mechanical 

 response of frog's nerve to undergo this staircase enhance- 

 ment. From this effect then it is easy to understand that an 



