270 ELECTRO-PHYSIOLOGY , HAP. 



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- 



c n arised at A is applied to 



a second nerve CD by 

 part of its length, the 



FIG. 205. 



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, i.e. 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 (peronazus and tibialis, Fig. 206). If one or the other- 

 is non - electrically excited, the muscles innervated from that 

 branch alone become active, never those from the other. If, 

 however, an electrical current is passed through the tibial branch 

 at a point not too remote from the bifurcation, contraction occurs 

 at make and break not merely in the muscle A but also in B, 

 which is supplied by the peronaeal branch, because the primary 



