416 ELECTRO-PHYSIOLOGY CHAP. 



be greater, in proportion as the induction current is weaker, and 

 the excitation intervals, with a given state of excitability, longer. 

 Often the secondary muscle only begins to twitch when the stimu- 

 lation of the primary muscle has already lasted some minutes, 

 and when, owing to fatigue, the changes of form in the latter, 

 corresponding with the single stimuli, are hardly to be recognised. 

 It is natural to suppose that this might be solely an effect of 

 summation in the secondary nerve, but that is easily excluded by 

 applying the nerve, not at the beginning of excitation in the 

 primary muscle, but after a greater or lesser number of stimuli. 

 Without exception the secondary twitches appear in full vigour 

 as soon as the nerve is brought into contact with the primary 

 muscle, showing that the gradual development of activity in the 

 latter depends upon changes in its substance produced by repeated 

 excitation. 



If in these two cases it appears highly probable that the 

 difference in the secondary action from muscle to nerve depends 

 upon the intensity of electrical action in the former, in other 

 instances disparity of time-relations, and form, of the wave of 

 electrical variation, seem to be the determining factors. Above 

 all there is the striking difference in secondary action from 

 muscle to nerve, when the former is directly excited in a variety 

 of ways. As a general rule it is harder to elicit secondary con- 

 tractions when the primary muscle is excited directly, than when 

 it is excited from the nerve. Du Bois-Eeymond in fact supposed 

 that there was no secondary twitch, if a wave of excitation was 

 set up in the sartorius or gracilis, with a sciatic nerve lying on the 

 excitable upper end of the muscle. Kiihne was the first to show, 

 on the contrary, that the twitch produced by moistening the 

 fresh section of a curarised sartorius with a conducting fluid 

 (which Hering proved to be electrical in character) is peculiarly 

 adapted to secondary action, a fact which only makes it more 

 remarkable that the direct electrical excitation of the same muscle 

 by artificial currents should be so ineffective for this purpose. 

 Kiihne did, indeed, observe unmistakable secondary action 

 on exciting one end of a muscle with single induction shocks, but 

 in all these cases such a strong current was needed that special 

 control experiments were required to exclude direct excitation of 

 the secondary nerve by current diffusion. If a battery current 

 is thrown in laterally by unpolarisable electrodes near one or 



