in ELECTRICAL EXCITATION OF MUSCLE 191 



current is much increased, the effect on the latent period, 

 though pronounced at first, may disappear completely. In the 

 opening excitation the latent period, is as a rule, longer than at 

 closure, so that with weaker battery currents the difference, as 

 compared with induction twitches, is even more marked than 

 at closure. But with increased current intensity and prolonged 

 closure, it may he almost entirely abolished in this case also. 

 In inquiring into the cause of the shorter latent period in make 

 and break induction twitches, the answer comes to hand when 

 we remember that in order to provoke a " twitch," a certain 

 acceleration of increase in the intensity of current in the muscle is 

 essential. According to the law proposed by du Bois-Eeyniond, 

 the electrical current does not excite by its absolute density, but 

 by its relative changes from one moment to another, the in- 

 centive to movement consequent on these changes being the more 

 considerable in proportion with their rapidity at uniform magnitude, 

 or brief duration in a time-unit. 



And if, on the other hand, we have reason to suppose that 

 constant currents of medium strength undergo slower alterations 

 of density within the muscle than induction currents (in conse- 

 quence of lower potential), then the longer latent period, at least 

 for closure twitches, would, as Tigerstedt points out (I.e. p. 197), be 

 dependent upon purely physical factors, and the self-evident con- 

 sequence of du Bois-Eeymond's " general law," as above stated. 

 But we have already shown that the first half at least of this law 

 has no application to muscle, and we shall subsequently find that 

 the second half is not generally applicable either. This does not 

 indeed cancel the possibility of explaining the above differences 

 in the latent period as indicated. 



Here, we are obviously dealing with the commencement of 

 the contraction only, not with its final magnitude and further 

 process. Although the excitatory effect of short, induced cur- 

 rents is certainly less than that of battery currents so far as 

 regards magnitude and duration of the twitch, it is easy to show 

 that the degree of current density required to produce a twitch, 

 however small, is more easily arrived at with induced, than with 

 constant, currents. 



This leads us directly to the question of the dependence <>f 

 .<-<-it<tt(>r/j effects upon the distribution in time of the elcctrieal move- 

 ment. On comparing contractile substances collectively, we are 



