Page:Life in Motion.djvu/122

 LECTURE IV Action of a nerve— Rapidity of nerve-current— Nature of nerve-current— Analogy with electric current— Nerveless animals— Heat in muscle— Muscles liberate mechanical energy and heat— Chemical changes in muscle.

have now seen that the living matter forming a muscle is irritable or excitable, that is to say, it responds or reacts to a stimulus. We have also learned that the muscle shows its response or reaction by a contraction. Lastly, we have found that the natural stimulus that sets the muscle into action is something that happens in a nerve. Let us to-day, in the first place, study more carefully than we have yet done what occurs in a nerve.

A nerve, like a muscle, is composed of living matter, and this living matter, like all living matter, is irritable; but it does not show its irritability in any way evident to our senses. Suppose I irritate a little bit of nerve, which I have every reason to think is still alive,