238 ELECTRO-PHYSIOLOGY CHAP. 



excitation is best effected by a descending current, and vice versa. 

 If, on the contrary, the nerve current and exciting currents are 

 heterodromous, the action of the exciting current will be dimin- 

 ished or abolished" (Griitzner, l.c.). Fleischl (15) subsequently 

 tried to overthrow this interpretation on the ground that effects 

 corresponding to his " law of contraction " might also be ob- 

 served in nerves of which the P.D. was compensated by an 

 artificial current. This, however, was answered by Griitzner and 

 Hermann, who pointed out that compensation can only neutralise 

 the branch of the current that flows in the applied circuit, and 

 not the internal P.D. of the nerve (or muscle), with its corre- 

 sponding currents. 



A remarkable case of interference between nerve and muscle 

 currents is that cited by Hering (11), in which the tetanus 

 consequent 011 division (supra) is absent, even in preparations 

 from highly susceptible cooled frogs, on dividing the thigh with 

 a single cut, when the current from the divided muscle, ascending 

 in the nerve, acts upon the latter so as to neutralise its current. 



As in striated muscle (sartorius) denervated by curare, "spurious 

 break twitches " may result from interference between demarcation 

 current and artificial exciting current, so likewise in nerve. The 

 striking dependence of the opening excitation upon the proximity 

 of the anode to an artificial section of the nerve has already been 

 insisted on. It is highly probable, if not proven, that these 

 opening twitches from the cross -section are not true opening 

 twitches, but closure contractions from the nerve current previ- 

 ously compensated in the leading-off circuit i.e. that this effect 

 is wholly analogous with the " false opening twitches of injured 

 muscle " (Hering, Griitzner, 11). 



If a rheochord is employed to send a branch of the current 

 from a battery through a nerve resting by its transverse and 

 longitudinal sections upon unpolarisable electrodes, the closure or 

 opening of the circuit being effected by a key introduced between 

 rheochord and electrodes, there may in favourable cases (Hering, 

 11) be a contraction, both on closing and on opening the "nerve 

 circuit " (which short-circuits the demarcation current externally), 

 even when the rheochord is not in the circuit. If this is effected, 

 and a key introduced, together with a reverser, into the same 

 (" battery ") circuit, the excitatory effects must differ (when the 

 deriving current from the battery is ascending in the nerve, and 



