The muscles, representing the contractile tissue of the body, are the scene of a most extensive oxidative process during the time of their contractile activity. Furthermore, inasmuch as the body heat is for the most part liberated by oxidation of sugars and fats in the muscles, and inasmuch as body heat is continuously liberated throughout the whole twenty-four hours, it becomes evident that the muscles are the scene of a continuous oxidation, which never ceases from the beginning of life until these tissues enter a condition of rigor mortis after death. The rate of oxidation is, however, far more active during muscular work than during rest.

The oxidative changes in the muscles consist in a simple oxidation of sugars and fats into carbon dioxid and water, with a liberation of the latent heat. The oxidation of proteins in the muscles results in the formation not only of carbon-dioxid gas and water, but also of a long list of nitrogenous mid-products. These mid-products pass in the blood to the liver, where they are further oxidized and prepared for excretion as described above. As glycogen has been found in muscle tissue, it is supposed that when sugar comes to the muscles in larger quantities than can be utilized, a portion is changed to glycogen and deposited for later use. The chemical change in this process of glycogenosis and glycolysis is parallel to similar changes which take place in the liver.