of the two points, say B, more or less permanently
irresponsive. In that case, stimulus will cause greater electrical
disturbance at the more responsive point, say A, and this will be shown
by the galvanometer as a current of response. To make B less responsive
we may injure it by means of a cross-sectional cut, a burn, or the
action of strong chemical reagents.

[Illustration: FIG. 2.--ELECTRIC METHOD OF DETECTING NERVE RESPONSE
(_a_) Iso-electric contacts; no current in the galvanometer.
(_b_) The end B injured; current of injury from B to A: stimulation
gives rise to an action current from A to B.
(_c_) Non-polarisable electrode.]

#Current of injury.#--We shall revert to the subject of electric response;
meanwhile it is necessary to say a few words regarding the electric
disturbance caused by the injury itself. Since the physico-chemical
conditions of the uninjured A and the injured B are now no longer the
same, it follows 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 _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