BY DR. MICHAEL FOSTER. 383 



Then reopen a a'. The needle will be found to return to the 

 position it had in Obs. I. 



Obs. III. Repeat the observation, but reverse the polarizing 

 current; let it flow from p' to p, that is, in a direction con- 

 trary to the natural nerve-current. The needle of the galvano- 

 meter will now be found to have suffered a diminution of de- 

 flection instead of an increase. 



" When a constant current is allowed to break into a nerve, 

 the natural nerve current, even at some distance from the 

 electrodes, is affected during the whole lime of the passage of 

 the constant (polariziag) current; when the natural and 

 polarizing cur rents have the same direction, the natural cur- 

 rent is increased ; when contrary directions, the natural cur- 

 rent is diminished." 



This condition of the nerve, maintained during the whole 

 passage of the current, is known as electrotonus. 



Obs. IV. Tie the nerve very tightly with a ligature between 

 the polarizing and the galvanometer electrodes ; or divide it, 

 and place the ends carefully in exact opposition, and repeat 

 the observations. It will be found that the natural current is 

 in no way affected by the polarizing current. 



The phenomena, therefore, are not due to any escape of the 

 battery current : something more than mere physical continu- 

 ity is required for their development. 



Obs. V. Repeat the observations, placing the thinner (peri- 

 pheral) end of the nerve on the galvanometer electrodes, and 

 the thicker on the polarizing electrodes. 



The results are the same ; electrotonus is established equally 

 well in either direction. 



Obs. VI. The same result may b^ better shown in the fol- 

 lowing way: Take three pair of electrodes. Place the polar- 

 izing pair p p', fig. 290, in the middle of the nerve, and connect 

 the other two pair with the two cut ends, as shown in the figure. 

 Bring the wires from a a' to a key, and those from b b f to 

 another key; then the wires from both keys to the same bind- 

 ing screws of the galvanometer. By opening the key of a a' 

 while that of b b r is shut, or vice versa, the amount of natural 

 current in a a' or 6 b' may be respectively determined. (Or 

 use the double key, as directed in Chap. XIX., sec. IX.). 

 Determine both before the key of p p' is opened. Then open 

 the key of p p f and determine the amount of deflection both in 

 a a,' and b &'. 



It will be found that when the current passes from p to p' 

 in the direction of the arrow, as in the figure, the current at 

 b b f is diminished (in the neighborhood of the kathode), while 

 that at a a' is increased (in the neighborhood of the anode). 

 If the direction of the polarizing current be reversed, if it be 



