Response in the Living and Non-Living | Page 5

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 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.
#Action current.#--We have seen that a current of injury--sometimes known as 'current of rest'--flows in a nerve from the injured to the uninjured, and that the injured B is then less excitable than the uninjured A. If now the nerve be excited, there being a greater effect produced at A, the existing difference of potential may thus be reduced, with a consequent diminution of the current of injury. During stimulation, therefore, a nerve exhibits a negative variation. We may express this in a different way by saying that a 'current of action' was produced in response to stimulus, and acted in an opposite direction to the current of injury (fig.?2, b). The action current in the nerve is from the relatively more excited to the relatively less excited.
#Difficulties of present nomenclature.#--We shall deal later with a method by which a responsive current of action is obtained without any antecedent current of injury. 'Negative variation' has then no meaning. Or, again, a current of injury may sometimes undergo a change of direction (see note, p.?12). In view of these considerations it is necessary to have at our disposal other forms of expression by which the direction of the current of response can still be designated. Keeping in touch with the old phraseology, we might then call a current 'negative' that flowed from the more excited to the less excited. Or, bearing in mind the fact that an uninjured contact acts as the zinc in a voltaic couple, we might call it 'zincoid,' and the injured contact 'cuproid.' Stimulation of the uninjured end, approximating it to the condition of the injured, might then be said to induce a cuproid change.
The electric