Page:Response in the Living and Non-Living.djvu/31

 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 disturbance this current of injury remains approximately constant, and is therefore sometimes known as ‘the current of rest’ (ﬁg. 2, b).

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 connected by a strip of cloth 5 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. 8, a) and from A to B through the electrolyte .9. Or instead of connecting A and B by a single strip of cloth .9, we may connect them by two strips 3 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 .9 s’ to B, the wire W being interposed, as it were, in the electrolytic part of the circuit. If now a galvanometer be interposed at O, the current will ﬂow from B tOA through the galvanometer, i.e. from right to left. But if we interpose the galvanometer in the electrolytic 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. 8, d, where A B is a piece of nerve of which the B end is injured. The current in the galvanometer