382 ELECTROTONUS. 



The negative variation travels along the neroe in either di- 

 rection. 



O6.s. V. Ascertain as before, by ligature, that the effects 

 witnessed are not due to any escape of current. 



CHAPTER XXVII. 

 ELECTROTONUS. 



Ohs. I. Prepare as long a piece of nerve as possible. Get 

 ready tirv pair of non-polarizable electrodes. Place the thicker 

 (central) end of the nerve, on one pair of electrodes, a a' fig. 

 290. This figure is intended to represent diagrammatic-ally 

 the effects of a polarizing current, p p', acting on the centre 

 of a piece of nerve, as seen by testing either end with a gal- 

 vanometer. It will serve, however, to illustrate the simpler 

 case of Obs. I., if the electrodes 6 b' be supposed to be removed 

 and p p' brought nearer to that end of the nerve. Let one 

 electrode be on the transverse section of the nerve, and the 

 other on the longitudinal surface at some distance, so as to 

 obtain a tolerably good current. Connect this pair of elec- 

 trodes with the galvanometer, putting in a key or using the 

 shunt. 



Place the other end of the nerve on the other pair of elec- 

 trodes, p p' ; connect these electrodes with a cell, which may 

 be called the polarizing cell, interposing a commutator (Chap. 

 XIX., sec. VII.). Cover the nerve with a shade, or put it with 

 the electrodes in the nerve chamber (Chap. XIX., sec. IV.), to 

 protect it against evaporation. 



Both keys being down, and the needle of the galvanometer 

 being at zero, open the key of a a', and note the deflection of 

 the needle. The current will of course pass through the gal- 

 vanometer in the direction of the arrow in the figure from a to 

 ', and the circuit may be supposed to be completed by the 

 current passing inside the nerve in the direction of the arrow. 

 Shut the key of a a'. 



Obs. II. Now open the commutator of the polarizing cell in 

 such a way that the current of the cell passes from p to //, in 

 the direction of the arrows in the figure, i. e., flows in the same 

 direction as the natural nerve current flows through the gal- 

 vanometer. Open the key of a a'. Note again the deflection 

 of the needle; it will be found to be greater than it was before. 



Shut the key of a a' and shut off the polarizing current. 



