Page:Life in Motion.djvu/49

Rh and the hammer of the bell is kept up. This condition you will notice is not a rapid, sudden twitch, but a slow, steady, persistent pull or contraction. The muscle has passed into a state of cramp, or, as physiologists term it, a state of tetanus. The short, sudden contraction we shall call a twitch or simple spasm.

We repeat the experiment, only irritating the muscle instead of the nerve, and we get the same result: a single twitch with a single shock and tetanus when the shocks come in rapid succession.

We have learned from this experiment, then (1), that when we irritate the nerve going to a muscle, the muscle becomes shorter and thicker, or, in other words, it contracts; (2), that a single shock of electricity to the nerve is followed by a sudden sharp twitch, a single contraction; and (3) that a number of shocks sent in rapid succession to the nerve causes the muscle to pass into a more lasting state of contraction called tetanus or cramp.

In the experiment we have just performed all the work the muscle did was to pull up the signal and ring the bell. It does not require much energy to do this, and the experiment