. Electro-physiology. Electrophysiology. •270 ELECTRO-PHYSIOLOGY electrotonus takes no appreciable time, since it appears at full strength immediately after closing the polarising current, and can be demonstrated with even the most fugitive induction currents, Helmholtz was the first to prove the same fact by means of the physiological rheoscope (31). In view of the known sensibility of the latter to much weaker currents than are here under consideration, it is quite intelligible that the electrotonic incremental current should be adequate to excite the nerves of a rheoscopic frog's leg
. Electro-physiology. Electrophysiology. •270 ELECTRO-PHYSIOLOGY electrotonus takes no appreciable time, since it appears at full strength immediately after closing the polarising current, and can be demonstrated with even the most fugitive induction currents, Helmholtz was the first to prove the same fact by means of the physiological rheoscope (31). In view of the known sensibility of the latter to much weaker currents than are here under consideration, it is quite intelligible that the electrotonic incremental current should be adequate to excite the nerves of a rheoscopic frog's leg, if properly led through it. It is even possible (as du Bois-Eeymond showed) to throw a superposed nerve into secondary electrotonus, by the electrotonic incremental current of the first nerve. If one end B (Fig. 205) of a medullated nerve pol- arised at A is applied to a second nerve CD by part of its length, the second nerve at once falls into a state of electrotonus; the end D is, however, at the opposite phase to B, since the incremental current at B flows through the end C of the applied nerve, which forms a shunt circuit in the opposite direction. If this nerve is still connected with its muscles, then both at make and, under favourable cir- cumstances, at break of the polarising current there will be a secondary contraction, which is not to be confused with the true secondary twitch from nerve to nerve caused by the current of action, as first demonstrated by Hering. Du Bois-Reymond's " paradoxical twitch " is a very interesting form of this secondary contraction depending on electrotonus. The conditions for the discharge of this secondary twitch by the electrotonic incremental current are especially favourable when the fibres of the two nerves, in so far as they are in juxtaposition, are as it were coherent, fused into a single stem. This is the case in the frog's sciatic with the two branches into which it divides at the knee (peronceus and tibialis,
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