become in the case of the muscle, mechanical energy and heat. This locked-up energy is liberated by one familiar process, that of burning. But burning is oxidation. The elements of the substance to be burnt are torn from each other, and they unite with the oxygen of the air to form simpler bodies. When this occurs, energy appears as heat. Similar phenomena occur in a muscle. The muscle needs oxygen and it needs food stuffs. It builds some of these food stuffs into its own substance, thus always making up for the "tear and wear" that goes on when it works, and it uses some of the others, effecting in some mysterious way chemical changes in them, always splitting them up into simpler bodies.
Let us think of the muscle as a little machine or engine. When it works, it is subjected to tear and wear, like any other machine, and by and by it would become unfit for work. This happens, as we all know, with any machine. The boiler plates of an engine get thinned, the pinions become slack, the wheels do not work so smoothly as at first, and a time comes when the engine becomes only old iron and is unfit for use. But
