Page:Life in Motion.djvu/128

108 breaks the circuit, and the muscle again contracts, but this time it describes on the smoked surface the curve B, seen to the left of A, in the diagram. You observe this curve leaves the horizontal line at B, that is, a little later than when the nerve was stimulated close to the muscle (Fig. 54).

It follows, therefore, that the distance on the horizontal line from A to B represents the time occupied by the transmission of the nervous impulse from B to A of the nerve. We measure the rate at which the glass plate was travelling by bringing to bear on it a marker connected with one of the prongs of this tuning-fork, and we cause the fork to vibrate at the moment when the glass plate dashes past the markers (Fig. 55). The time waves thus accurately measure the rate of movement of the glass plate, and consequently the minute interval of time between A and B.

This experiment proves that when a nerve is irritated at any point some kind of change is then produced, and that a change is propagated with a certain velocity along the nerve. This something we call a current, for want of a better term; but it is not a current