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 sh