RESPONSE OF NERVE 723 



positive response. The interference of this with the normal 

 response at the uninjured contact is thus apt to give rise to 

 various complications. In contrast with this we have the 

 reliability of the mechanical response of the nerve, in which 

 the effect recorded is direct, and not differential. Again, the 

 electrical form of stimulus, which is almost universally 

 employed for the excitation of nerve, is liable by leakage, 

 unless very great precautions are taken, to vitiate the results 

 obtained by the electrical mode of response. When the 

 response observed, however, is not electrical but mechanical, 

 this source of error is obviously eliminated. 



By means of mechanical response, the molecular trans^ 

 formations through which the nerve passes, under the action 

 of stimulus itself, may be observed with the greatest clearness. 

 An isolated nerve, cut off from its natural supply of energy, 

 generally falls into a sub-tonicity indicated in the mechanical 

 record, as an increasing abnormal relaxation ; and the 

 application of stimulus induces at this point an abnormal 

 positive response, of sudden expansion. Successive or con- 

 tinuous stimulations, however, transform the nerve from 

 condition A to condition B, the abnormal expansion being 

 arrested and converted into increasing contraction. During 

 this stage, then, the responses to individual stimuli are trans- 

 formed from the abnormal expansive positive to the normal 

 contractile negative, through an intermediate diphasic. 

 Molecular transformation is here very rapid and the re- 

 sponses show a staircase increase (fig. 382). An intervening 

 period of tetanisation will now have the effect of enhanc- 

 ing the response (fig. 383). In the clear demonstration 

 thus obtainable of a progressive molecular transformation, 

 with corresponding variations of response at its different 

 stages, we arrive at the true explanation of the change from 

 the abnormal positive to the normal negative, in electrical 

 response, and also of the enhancement of the normal negative 

 after an intervening period of tetanisation (figs v 275-278). 



The next stage to be reached is C, where the responses 

 are uniform. After this, we arrive at D, where fatigue-decline 



3 A2 



