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. ‘ Nega- tive 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 expres-
