x ELECTROMOTIVE ACTION IN XERA T E 203 



of conduction in nerve, the interval at which the wave passes 

 the two leading-off contacts is too small to be analysed by the 

 rheotome, even when the electrodes are far apart ; while on 

 extending the tract led off, the resistance, which is already con- 

 siderable, increases so much that the effect becomes imperceptible. 

 Hermann, however, overcame these obstacles ; he depressed 

 the rate of conductivity by cold, and employed bundles of 4-6 

 sciatics. He was then able to obtain a distinct separation of the 

 two opposite currents with the rheotome, and thus established 

 the undulatory character of that alteration in the nervous sub- 

 stance, which in the galvanic expression of excitation is charac- 

 terised by negativity. 



FIG. 202. 



If one lead-off is at the artificial transverse section, the corre- 

 sponding phase fails, as in muscle, or is at any rate "rendered uncer- 

 tain." Hermann found, without exception, that the second phase 

 was less conspicuous and more prolonged than the first ; this is not, 

 however, due, as in muscle, to decrement of excitation, but 

 refers strictly to the fact that the first phase has not nearly 

 expired when the second is at its maximum. This is clear from the 

 accompanying diagram (Fig. 2 02, from Hermann). "The abscissa 

 of represents the times, positive ordinates the homodromous, 

 negative ordinates the heterodromous direction of current. The 

 curve Aaa gives the temporal relations of the action current from 

 the first lead-off, Bbb that of the action current from the second. 

 AB is the time required for transmitting the excitation between 

 the two leading-off contacts. Accc is therefore the curve of the 

 resulting diphasic current of action, the second phase (2) of 



