ELECTRICAL PHENOMENA. 103 



These figures will vary naturally for the nerves of different ani- 

 mals or for different nerves in the same animal, for it must always 

 be remembered that nerve fibers, whose functions in general are so 

 similar, differ much in obvious microscopical structure and probably 

 more widely in their chemical composition. Using an analogy that 

 is familiar, we may say that when a stimulus acts upon a living 

 nerve a wave of elect ronegativity spreads from the stimulated 

 spot and travels in wave form witl a definite velocity, just as water 

 waves radiate from the spot at which a stone is thrown into a quiet 

 pool. A similar phenomenon occurs in muscle fibers when stimu- 

 lated, but the negative condition travels over the muscle fiber at a 

 slower speed, 3 to 4 meters per second in frog's muscle, and with a 

 wave length, according to Bernstein, of only 10 mms. This wave 

 of negativity in the muscle begins during the latent period and, 

 therefore, precedes the actual shortening at any point. 



This phenomenon of a negative electrical condition traveling 

 over the nerve or muscle and giving us an action current when led 

 off through a galvanometer is of the greatest physiological impor- 

 tance, particularly in the study of nerves. It has been shown 

 that in the nerve this wave of negativity marks the progress of the 

 wave of excitation, and, since we can study its progress by means 

 of the galvanometer or capillary electrometer, we can thus study the 

 excitability and conductivity in nerves when removed from con- 

 nection with their end-organs. That the negative wave, or the 

 action current that it gives rise to, is an invariable sign of the 

 passage of an excitation or nerve impulse is shown by the facts 

 that it is absent in the dead nerve, and that in the living nerve it is 

 produced by mechanical,* chemical,! and reflex t stimulations, as 

 well as by the more usual method of electrical stimulation. 



Monophasic and Diphasic Action Currents. According to 

 the conception of the action current given above, it is evident that 

 it should be obtained upon stimulation when a living normal nerve 

 is connected at any two points of its course with a galvanometer or 

 capillary electrometer. The detection of the current under such 

 conditions offers more difficulties, because it is diaphasic, as will 

 be seen from the accompanying diagram (Fig. 46). The figure 

 represents a normal nerve led off to the galvanometer from two 

 points, b and c, of its longitudinal surface. As these points in the 

 uninjured nerve have the same potential, no current is shown by 

 the galvanometer. If the nerve is stimulated at a by a single 

 stimulus, a negative condition or charge passes along the nerve. 

 When it reaches the point b, there will be a momentary current 



* Steinach, "Pfliiger's Archiv," 55, 487, 1894. 

 tGriitzner, "Pfliiger's Archiv," 25, 255, 1881. 

 J Boruttau, "Pfliiger's Archiv," 84, and 90, 1901-1902. 



