Page:Collected Physical Papers.djvu/284

264 surface being in similar molecular conditions, their electrical level or potential will be the same. They are iso-electric. No current will be exhibited by the indicating galvanometer when two non-polarisable electrodes connected with it are applied to A and B. But if one of the two points, say B, be injured by a cut, or burn, then, the conditions of A and B being different, there will be a difference of electric level or potential between them, and a current will flow from the injured to the uninjured, that is from B to A (fig. 63). This current remains approximately constant as long as the muscle is at rest, and is for this reason known as "current of rest." As it is primarily due to injury, it is also known as "current of injury." If now the muscle be thrown into an excitatory state by stimulus, there will be a greater relative change at the uninjured A, and the original difference of electric level will be disturbed. There is then a negative variation or diminution of the original current of rest. This negative variation or "action current" constitutes the "response," the intensity of which increases with the intensity of the stimulus.

If a piece of muscle be taken, and simultaneous records of its response be made by the mechanical and electrical recorders, it will be found that the one is practically a duplicate of the other.

I find that the electric response seen in animal tissues is also strongly exhibited by the tissues of plants. Various parts of plants—leaves, stems, stalks, and roots