Page:Life in Motion.djvu/124

104 We may suppose that this time may be divided into three portions. First, a period in which changes occur in the nerve; second, a period in which changes occur in the muscle (the latent period); and third, a period occupied by the contraction of the muscle. Now suppose that instead of irritating the nerve close to the muscle, we irritate it at a point farther off, say two inches from the muscle. If, then, the times of the latent period and of the contraction remain the same, and if something travels alons: the nerve from the distant irritated point, the muscle should contract a little later than when the nerve is irritated close to the muscle. Reasoning in this way, Professors von Helmholtz and du Bois Reymond, now a good many years ago, devised methods by which this experiment may be made.

Let us try an experiment or two to illustrate this method. One of the most ingenious and simplest instruments for the purpose is the spring myograph of Professor du Bois Reymond, which I now show you. It consists of a smoked-glass plate, which is driven in front of the recording marker of the myograph by the recoil of a steel spring C. Underneath the