240 ELECTRO-PHYSIOLOGY CHAP. 



If the battery current enters at the transverse section of th3 

 nerve,' i.e. is descending, the effect, as pointed out by Hering, is 

 again dissimilar, according as the battery circuit is closed with 

 previous closure of the nerve circuit, or vice versa. " For in the 

 former case a current, viz. that of the nerve, is already entering at 

 the longitudinal electrode, so that the superposition of the branch 

 from the battery current will only increase it. But if the nerve 

 circuit is closed subsequent to closure of the battery circuit, the 

 nerve current and battery current will summate at the moment of 

 closure, and the effect of the latter is thus augmented. At break 

 of the nerve circuit, again, the two currents disappear simul- 

 taneously." " On commencing with minimal ingoing currents at 

 the cross-section of the nerve, the make twitch appears first at 

 closure of the nerve circuit, and only with stronger currents on 

 closing the battery circuit also. An analogous reaction may be 

 determined for the opening twitches " (Hering, 11). 



Du Bois-Keymond (Ges. Abkandl. i. p. 196) pointed out that, 

 under certain conditions, the effect is fundamentally different 

 according as the current is made or broken in the principal or 

 deriving circuit (battery or nerve circuit). But as metal electrodes 

 were exclusively employed in these experiments, there must have 

 been extensive external polarisation at the interface of the animal 

 tissues and the electrodes. 



A characteristic effect of interference between exciting and 

 nerve current is the " breach " in the series of opening twitches, 

 first pointed out by Griitzner (14). It may be observed when 

 any part of the nerve, in which there is a descending current (as, 

 e.g., at the cross -section), is excited with ascending battery 

 currents of increasing strength. Opening twitches then appear 

 even with very low intensities of current, increasing at first with 

 its augmentation, and then diminishing to zero, after which they 

 again increase in magnitude. 



The magnitude, i.e. E.M.F., of the current in a leading-oft 

 circuit, in which one contact is applied to the transverse 

 section of a nerve, while the other rests upon a point of the 

 longitudinal surface, of course depends to a large extent upon the 

 distance between the electrodes. The deriving current is experi- 

 mentally found to be greatest when this = 5 7 mm., when com- 

 pensation by an artificial heterodromous current will accordingly 

 be most adequate. The conditions are much less favourable with 



