all 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 change produced in a normal nerve by stimulation may
therefore be expressed by saying that there has been a negative
variation, or that there was a current of action from the more excited
to the less excited, or that stimulation has produced a cuproid
change.

The excitation, or molecular disturbance, produced by a stimulus has
thus a concomitant electrical expression. As the excitatory state
disappears with the return of the excitable tissue to its original
condition, the current of action will gradually disappear.[3] The
movement of the galvanometer needle during excitation of the tissue thus
indicates a molecular upset by the stimulus; and the gradual creeping
back of the galvanometer deflection exhibits a molecular recovery.

This transitory electrical variation constitutes the 'response,' and its
intensity varies according to that of the stimulus.

#Electric recorder.#--We have thus a method of obtaining curves of
response electrically. After all, it is not essentially very different
from the mechanical method. In this case we use a magnetic lever
(fig. 4, _a_), the needle of the galvanometer, which is deflected by the
electromagnetic pull of the current, generated under the action of
stimulus, just as the mechanical lever was deflected by the mechanical
pull of the muscle contracting under stimulus.

The accompanying diagram (fig. 4, _b_) shows how, under the action of
stimulus, the current of rest undergoes a transitory diminution, and how
on the cessation of stimulus there is gradual recovery of the tissue, as
exhibi