ADDENDUM. 547 



or reversal of the < demarcation current ' at the moment that the 

 excitation ceases. The main facts ascertained by Hering were 

 that effects appeared even when the tetanisation lasted only a frag- 

 ment of a second, and that when the excitation was prolonged in 

 successive experiments, the positive after effect increased up to 

 a duration of 30", but subsequently diminished. 



To these facts Mr. Head has added the following. He has first 

 shown that the positive after effect cannot be regarded as a second 

 phase of the excitatory effect produced by each single excitation, 

 for his rheotome observations show that the negative phase is fol- 

 lowed by a period of complete restoration of the previously existing 

 electrical state, which lasts for several hundredths of a second. 

 Secondly, he has shown that in short tetanisation lasting not more 

 than a few seconds, the excitatory effect, i. e. diminution of the 

 demarcation current, is nearly constant a result which could not 

 happen unless the positive effect were deferred. Thirdly, that in 

 comparing nerves prepared with the utmost possible care and pro- 

 tected from evaporation, with nerves in a less favourable state, it is 

 found that although the latter may show no inferiority or sign of 

 impaired functional activity as regards the negative variation, they 

 are inferior as regards the positive after effect, so that, in repeated 

 observations, the former remains unimpaired while the latter rapidly 

 diminishes. 



These facts he interprets to mean that the positive after effect is 

 in the nature of a reaction of the nerve, consequent on excitation, 

 i. e. a change in the opposite direction to the excitatory change, by 

 which it not merely returns to the normal, but passes into a condi- 

 tion on the other side of. the normal, so that the longitudinal 

 surface to which the electrode is applied becomes more positive 

 than it was before excitation. This view he considers to be con- 

 firmed by the observation that the effect in question is only observed 

 when the leading off contact on the longitudinal surface is at a 

 considerable distance from the injured end of the nerve. 



N U 2 



