168 ELECTRO-PHYSIOLOGY CHAP. 



of nerve, the breadth of the glass tube, is covered by it. 

 It is advisable to bring the whole preparation, with the electrodes, 

 into a moist chamber, to preserve the free end of the nerve from 

 drying in prolonged experiments. The muscle is connected with 

 a writing-point outside the chamber by means of a thread wound 

 round a cylinder, by which the changes of form are recorded on 

 the cylinder of a kymograph rotating at varying rapidity. On 

 exciting with weak currents a plain increase of effect from 

 closure will be observed after a few minutes, if the current leaves 

 at a part of the nerve treated with NaCl. The twitches, how- 

 ever, soon become tetanic, and after a short time there is a marked 



FIG. 188. Effect upon excitability, of local treatment with salt at the kathode. Transition from 

 descending closure twitch to closure tetanus. 



tetanus (Fig. 188) at every closure of the current in the direc- 

 tion indicated, which at first disappears again completely on 

 opening the circuit, but is persistent at later stages of the NaCl 

 effect, when of course further observations are impossible. At a 

 time when, after application of Nad to the electrode proximal to 

 the muscle, a weak descending current already discharges a 

 vigorous closure tetanus, the closure of the same current in the 

 opposite direction yields, as a rule, only a simple twitch, which 

 cannot in time-relations or magnitude be distinguished from 

 the closure twitches discharged under the same experimental con- 

 ditions by local application of NaCl. This fact is by no means 

 without interest, since it shows that, as regards the magnitude of 

 final result from the excitation of any point of the nerve, it is a 

 matter of indifference whether the " excitatory wave " discharged 

 passes through a tract of nerve already in a condition of heightened 

 excitability, or no. 



The opening of weak ascending currents has, as a rule, no 

 effect after local application of NaCl at the anode, although on 



