ELECTROMOTIVE ACTION IN NERVE 271 



electrotonic nerve throws the other nerve forming with it a com- 

 mon trunk into secondary electrotonus. Since even brief impacts 

 of current, or induction shocks, have an electrotonic 

 effect, it is clear that secondary tetanus may readily be 

 induced by tetanising the primary nerve. The excita- 

 tory action of both primary and secondary electrotonus 

 increases rapidly with approximation of polarised to 

 led-off tract (as might be anticipated from the marked 

 rise in potential near the polarised tract). In this we 

 have apparently (supra) a means of distinguishing 

 between the true secondary excitation from nerve to 

 nerve, and the paradoxical contraction (Hering, 11). 



Helmholtz made use of the latter as follows, in order 

 to determine the time occupied in establishing the gal- 

 vanic changes in electrotonus. A second, isolated nerve was applied 

 to a sciatic still in connection with the gastrocnemius, in such a way 

 that the half of the nerve proximal to the recording muscle was 

 brought into contact with the corresponding half of the other 

 nerve (Fig. 207). Electrotonic stimulation of corresponding 

 points at the central end of the two nerves yielded two twitches 

 in succession, one produced by direct excitation of the nerve to 

 the muscle, the other by secondary electrotonus. The secondary 

 contraction from the nerve was not found to enter perceptibly 

 later than the primary twitch, whence Helmholtz concludes that 

 " the electrotonic state does not make its appearance demonstrably 

 later than the electrotonic current which excites it," and does 

 not therefore, like excitation, require a measurable period in 

 which to diffuse over the extrapolar region. Du Bois-Reymond 

 (6, p. 258) had already pointed out that Helmholtz's experiment, 

 strictly speaking, can only mean that the changes fundamental to 

 electrotonus, and the process of excitation, are transmitted at 

 equal rapidity in the nerve. This follows directly from Hermann's 

 words (19, p. 162). "If the interval between excitation and 

 contraction of the muscle [M t Fig. 207] is the same, whether a 

 or b be excited, this proves that the electrotonus, in order to 

 spread in the first nerve from a to c, requires as much time as is 

 taken by the excitation to spread in the second nerve from b to 

 c'. But if the electrotonus at c is strong enough to excite the 

 second nerve, it will certainly produce direct excitation at c', at 

 least as strongly, when it is directly produced by excitation of b 



