Page:Life in Motion.djvu/86

66 we have only a horizontal line drawn thus—

Then Mr. Brodie, in the next place, closes the break, brings the carriage slowly up to it, and opens the break gently. The moment this is done, a shock (the opening shock) comes from the secondary coil and the muscle makes a mark a b thus—

He then again closes the break, brings up the railway to the catch, releases the catch, the railway dashes across, opens the break, the nerve gets the shock from the secondary coil, and the muscle contracts. But you observe the muscle has contracted a little later than the instant the break was opened, so that we get this curve (Fig. 31).

You notice the momentary twitch. Here is the tracing. You see the muscle curve has begun a little later than the mark made by the signal, that is to say, the muscle did not contract the instant the nerve got the shock. A little time intervened, something like the one-hundredth of a second, during which nothing visible happened,