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 _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 connecting 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'_ 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 to A 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. 3, _d_, where A B is a piece of nerve of which the B end is injured. The current in the galvanometer through the non-polarisable electrode is from left to right. The uninjured end is therefore comparable to the zinc in a voltaic cell (is zincoid), the injured being copper-like or cuproid.[2] [Illustration: FIG. 3.--DIAGRAM SHOWING THE CORRESPONDENCE BETWEEN INJURED (B) AND UNINJURED (A) CONTACTS IN NERVE, AND Cu AND Zn IN A VOLTAIC ELEMENT Comparison of (_c_) and (_d_) will show that the injured end of B in (_d_) corresponds with the Cu in (_c_).] If the electrical condition of, say, zinc in the voltaic couple (fig. 3, _c_) undergo any change (and I shall show later that this can be caused by molecular disturbance), then the existing difference of potential between A and B will also undergo variation. If for example the electrical condition of A approach that of B, the potential difference will undergo a diminution, and the current hitherto flowing in the circuit will, as a consequence, display a diminution, or _negative_ variation.