ELECTRIC RESPONSE 7 



that their electric conditions have also become different. 

 They are no longer iso-electric. There is thus a more or less 

 permanent or resting difference of electric potential between 

 them. A current the current of injury is found to flow 

 in the nerve, from the injured to the uninjured, and in the 

 galvanometer, through the electrolytic contacts from the 

 uninjured to the injured. As long as there is no further dis- 

 turbance this current of injury remains approximately con- 

 stant, and is therefore sometimes known as ' the current of 

 rest' (fig. 2,6). 



A piece of living tissue, unequally injured at the two ends, 

 is thus seen to act like a voltaic element, comparable to a 

 copper and zinc couple. As some confusion has arisen, on the 

 question of whether the injured end is like the zinc or copper 

 in such a combination, it will perhaps be well to enter upon 

 this subject in detail. 



If we take two rods, of zinc and copper respectively, in 

 metallic contact, and further, if the points A and B are con- 

 nected by a strip of cloth s moistened with salt solution, it 

 will be seen that we have a complete voltaic element. A 

 current will now flow from B to A in the metal (fig. 3, a) and 

 from A to B through the electrolyte s. Or instead of connect- 

 ing A and B by a single strip of cloth s, we may connect them by 

 two strips s s', leading to non-polarisable electrodes E E'. The 

 current will then be found just the same as before, i.e. from 

 B to A in the metallic part, and from A through s s f to B, the 

 wire w being interposed, as it were, in the electrolytic part of the 

 circuit. If now a galvanometer be interposed at 0, the current 

 will flow from B to A through the galvanometer, i.e. from right 

 to left. But if we interpose the galvanometer in the electro- 

 lytic part of the circuit, that is to say, at w, the same current 

 will appear to flow in the opposite direction. In fig. 3, c, the 

 galvanometer is so interposed, and in this case it is to be 

 noticed that when the current in the galvanometer flows from 

 left to right, the metal connected to the left is zinc. 



Compare fig. 3, d, where A B is a piece of nerve of which 

 the B end is injured. The current in the galvanometer 



