Page:Life in Motion.djvu/101

Rh it is produced by about 19.5 vibrations per second. If so, it follows that in a persistent muscular contraction in a healthy person, the muscle must be vibrating or quivering that number of times per second; or, in other words, the contraction is a kind of tetanus produced by about twenty shocks per second. But the stimulus that causes a voluntary contraction comes from the nervous system, the impulse originally starting from the brain. This stimulus passes along the nerves to the muscles and is their normal stimulus; but these considerations show us that the nervous stimulus, whatever it may be, is not like a continuous current flowing from the brain in the nerves to the muscles, but that it is intermittent and is more comparable to a series of shocks sent out at the rate of from ten (as some hold) to about twenty per second. Thus you see the study of tetanus lets us get a glimpse of what is probably occurring in every voluntary movement.

We have seen that a muscle is contractile. Has it any other special properties? Here are two muscles hanging side by side of about equal size and equal weight. Each has a strong silk thread tied round its tendon and a