ELECTRICAL PHENOMENA. 97 



the example given, relative to that of the demarcation current is as 

 10 to 50 ; so that frequently the phenomenon of the negative varia- 

 tion is known also as the action current. The explanation given 

 for this action current is that the nerve or muscle when excited 

 takes on an electrical condition which is negative as regards any 

 unexcited or less excited portion of the nerve. The effect upon the 

 demarcation current is illustrated in the accompanying diagram. 

 The demarcation current in a nerve is led off to a galvanometer 

 by electrodes placed at b and c. When the nerve is stimulated at 

 a the excitation set up passes along the nerve, and wherever it may 

 be that portion of the nerve is thrown into an electronegative con T 

 dition. When this condition reaches a point where it can influence 

 the galvanometer that is, when it reaches 6, it will diminish the 

 difference in potential that exists between b and c, and therefore 

 reduce the current 

 flowing from b to c. 

 Bernstein* has 

 shown that this neg- 

 ative condition 

 moves in the form of 

 a wave. ^That is, at 

 any point the nega- 

 tivity grOWS tO a Fig 42 _ Schema to i nd i cat e the method of detecting 

 maximum and tVipn the action current in a stimulated excised nerve: b and c, 

 611 the leading off electrodes, one 9 n the longitudinal, one on 



diminishes. M.Ore- * Q e cut surface; the demarcation current passes through 

 . the galvanometer, g, in the direction of the arrows; a, stimu- 



, it travels at a lating electrodes from induction coil ; the stimulus causes a 

 i i negative condition, which passes along the nerve; when 



Velocity this reaches 6 it causes a partial reversal of the demarca- 

 is P fl S i 1 V tion current gi ym S tne negative variation or action cur- 



measured. Accord- 

 ing to his experiments, the velocity of this wave in the frog's 

 motor nerve is from 25 to 28 meters per second, and the length 

 of the wave is about 18 mms. Hermann, on the contrary, be- 

 lieves that, in the excised nerve at least, the length of the wave 

 may be greater, reaching perhaps 140 mms. 



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 electronegativity spreads from the stimulated 

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

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



* Bernstein, " Untersuchungen iiber den Erregungsvorgang im Nerven- 

 und Muskelsysteme," Heidelberg, 1871. 



7 



