ELECTROMOTIVE ACTION IN NERVE 249 



conditions (more especially in preparations of cooled Temporaries) 

 the positive deflection not infrequently exceeded the previous 

 negative variation. In E. esculenta, according to Head, the average of 

 maximal positive variations was something over 5 per cent that of 

 the largest negative effects ; in R. temporaries it was over 8 1 per cent. 

 In " warm " frogs kept for several days in a room where the temper- 

 ature did not fall below 15 C. even at night, the positive after- 

 variation invariably failed, in spite of a marked negative variation. 

 This seems to be due to some alteration at the longitudinal lead- 

 off, developed at the close of the stimulation, and opposed in its 

 electrical relations to the effect of excitation. It may perhaps be 

 regarded as the galvanic expression of some reaction of the 

 excitable substance from the previous excitation (as negative 

 variation upon the galvanometer), a process of restitution that 

 only appears fully under favourable conditions. 



From this point of view it is intelligible that the positive 

 after-variation should increase within certain limits with the 

 duration of the previous excitation, as also that with repeated 

 stimulation the positive should decline before the negative effect. 

 For the potential reaction in the nerve must be essentially reduced 

 by persistent activity, supposing this to be accompanied by definite, 

 even if minimal, metabolism. The " non-fatiguability " of medullated 

 nerve proves this to occur only in the lowest possible, and not 

 directly demonstrable, degree. The positive after-effect (or, more 

 correctly, its absence) would accordingly be the only certain 

 criterion of fatigue in nerve -substance. "The more or less 

 exhausted nerve is characterised less by weakened response to 

 excitation, than by failure to give the opposite reaction at the 

 cessation of the stimulus, as energetically as the fresh nerve. The 

 vigour of this reaction, as expressed in the positive (after) varia- 

 tion, is the measure of energy in the nerve " (Head). 



In view of the extraordinary resistance of medullated nerve, 

 whether cold- or warm-blooded, to the total interruption of its 

 normal nutritive relations (supra), it is scarcely surprising that 

 the negative variation should appear on the galvanometer as the 

 expression of excitation (like the excitation itself in the nor- 

 mally-nourished and natural end-organ), long after the nerve has 

 been isolated. Hermann (19) often observed galvanic manifesta- 

 tions of excitation in rabbit's nerves for several hours after 

 the muscle had ceased to respond, and had indeed lost its 



