106 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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

 When it reaches the point b, there will be a momentar}- current 

 through the galvanometer from c to 6; as the charge passes on 

 to c, this point in turn will become negative to b, and there will 

 be a momentary current through the galvanometer in the other 

 direction. The diphasic current that occurs under these con- 

 ditions cannot be detected by the ordinary galvanometer, even 

 when a series of stimuli is sent into the nerve at a, since the 

 movable system in this instrument has too much inertia to 

 respond to such quick changes in opposite directions. With 

 the more mobile string-galvanometer or capillary electrometer 

 the diphasic currents have been demonstrated successfully. In 

 laboratory investigations one of the leading off electrodes, c, 

 is usually placed on the cut end of the nerve. Under this con- 

 dition the action current becomes monophasic and shows itself 

 as a negative variation of the demarcation current. This 

 difference is due to the fact that the negative condition accompany- 

 ing or constituting the wave of excitation undergoes a decrement 

 as it enters a region in which a negative condition already pre- 

 vails. Therefore, when the leading-off electrodes are placed so 

 that one is on the longitudinal and one on the cut surface, the 

 change of potential accompanying the excitation will affect only 

 the first electrode (6) and give a monophasic variation, which can 

 now be shown by the usual galvanometer, provided a series of 

 stimuli is thrown in at a. 



Fig. 47. — Schema to show the arrangement for obtaining a diphasic action current. 

 The arrangement differs from that in Fig. 46 only in that both leading off electrodes, b and 

 c, are placed on the longitudinal surface. No demarcation current is indicated. When 

 the nerve is stimulated at a the negative charge reaches b first, causing a current through 

 the galvanometer from c to b. Subsequently it reaches c and causes a second current 

 in the opposite direction from b to c. 



The Positive Variation. — It happens not infrequently that when one 

 electrode is placed upon the cut end, the nerve upon stimulation with a series 

 of induction shocks gives a positive instead of a negative variation of the 

 demarcation current. This resalt is usually explained as being due to a pre- 

 dominance of the anelectro tonic currents (see below). When this phenomenon 

 occurs it can usually be avoided by making a fresh section at the end of the 

 nerve. 



Detection of the Action Currents by the Rheoscopic Frog 

 Preparation or by the Telephone. — The motor nerve of a nerve- 

 muscle preparation from a frog is so extremely irritable to electrical 



