ELECTRICAL PHENOMENA. 99 



rent through the galvanometer in the other direction. The diphasic 

 current that occurs under these conditions cannot be detected by 

 a 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 capillary electrometer the diphasic currents 

 have been demonstrated successfully. In laboratory investiga- 

 tions one of the leading off electrodes, c, is usually placed on the 

 cut end of the nerve. Under this condition the action current be- 

 comes monophasic and shows itself as a negative variation of the 

 demarcation current. This difference is due to the fact that a nega- 

 tive condition upon excitation depends upon a living condition 

 of the nerve, and it can not, therefore, affect the nerve at the 

 electrode c if this latter is placed upon the cut end where the nerve 

 is dead or dying. It will affect only the electrode 6, and give only 

 the monophasic current, which can now be shown by the galvanom- 

 eter provided a series of stimuli is thrown in at a. 



Fig. 43. Schema to show the arrangement for obtaining a diphasic action current. 

 The arrangement differs from that in Fig. 42 only in that both leading off electrodes, 6 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 6. Subsequently it reaches c and causes a second current 

 in the opposite direction from 6 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 result is usually explained as being due to a pre- 

 dominance of the anelectrotonic currents (see below), but Wedenski has con- 

 tended recently that it is due to a peculiar condition of excitation in the nerve 

 at the cut end, a condition to which he gives the name of parabiosis. When 

 this phenomenon occurs it can usually be removed 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 

 currents that it may be used instead of a galvanometer to detect 

 the action currents in a stimulated muscle. A nerve-muscle prep- 

 aration used for this purpose is known as a rheoscopic preparation. 

 The way in which it is used is indicated in the accompanying 

 diagram, b represents the rheoscopic preparation, its nerve being 

 laid upon the muscle whose currents are being investigated, a, so as 



