290 ELECTRO-PHYSIOLOGY CHAP. 



galvanic current, the ascending portion only of the superposed 

 curve of variation is concerned in the excitation of the muscle, 

 this is by no means the case when the two interfering currents 

 are opposite in direction. Here, under some conditions, the de- 

 scending as well as the ascending portion of the curve may have 

 an excitatory effect (Griitzner, cf. Pftugers Arch, xxviii. p. 146) ; 

 in the first case excitation would occur on opening, in the second 

 on closing the circuit. With lower intensity of the polarising 

 galvanic current, the first effect would not, however, come into 

 consideration. But the other would also remain ineffective if the 

 battery current is so weak that its closure per se produces no 

 visible twitch. So long as the deepest point of the curve of 

 variation has not reached, or only just reaches, the abscissa, the 

 sudden renewal of the momentarily weakened, or interrupted 

 battery current will not induce excitation. It is only when the 

 deepest point of the curve of variation extends below the line of 

 the abscissa, i.e. when the intensity of the exciting current is so 

 great that it not merely interrupts the polarising constant cur- 

 rent, but a certain fraction of it also traverses the muscle in a 

 direction opposed to the constant current, that a twitch may 

 possibly follow, and to this it must be added that the excitatory 

 process will in this case be discharged at spots that were formerly 

 anodic. Excitation therefore occurs at the points where the 

 constant current enters, not during its passage, but at a minimal 

 interval after it has been broken. The return of the polarising 

 current to its original height, which follows immediately after, 

 will not accordingly produce excitation, being too low in intensity. 

 It is easy to see that with greater strength of the battery current, 

 the relations will become yet more complicated, since both its 

 negative variation of intensity, and also its recovery after previous 

 diminution or interruption, may cause excitation. 



It follows from the above that the possibility of producing 

 any changes of excitability in anodic points of the fibres by 

 means of an induction current opposed in direction to the 

 polarising current, is connected with very peculiar conditions. 



In the first place, it appears to be essential that the intensity 

 of the exciting current should considerably exceed that of the 

 polarising current, for it is only under these conditions that it is 

 possible to conclude with any probability that, during the closure 

 of the latter, that fraction of the induced current remaining for 



