What chemical changes occur in muscle when it contracts? What is the chemical source of its power? Carbonic acid is given off. This is the only product which we can collect and measure; but it is taken for granted that hydrogen atoms also combine with oxygen, forming water. There is no reason for thinking that nitrogen is removed from the molecules of its protoplasm with any greater rapidity during the activity of muscle than when it is quiescent (_cf._ p. 212). Is the oxidation immediate and complete, or does it occur in stages? For many years attention has been directed to lactic acid, partly because this substance is found in muscle which has been made to contract under experimental conditions, partly because, on theoretical grounds, glycogen (animal starch) is looked upon as the most important of muscle-foods. Lactic acid—C₃H₆O₃—has the same percentage composition as glycogen—C₆H₁₂O₆. Its formation from glycogen merely involves a rearrangement of atoms. It has been supposed that lactic acid is formed in the first instance, and then, if the supply of oxygen be sufficient, oxidized to carbonic acid and water. But this hypothesis may be resisted on various grounds. Undoubtedly, lactic acid appears when oxygen is deficient. Under all circumstances and in all tissues a certain amount of it is formed. There are reasons for thinking that it carries away the nitrogen which is wasted, as lactamide. But it does not follow that under normal conditions, when muscle is abundantly supplied with blood, lactic acid appears in any greater quantity during activity than during rest. The hypothesis is due to the misconception which we have already endeavoured to correct. It is difficult to get away from the steam-engine analogy. A steam-engine is made of iron and brass. These materials are subject to wear and tear; but they are not the source of its power. Its power is due to the combustion of fuel. Muscle, physiologists formerly said, is made of protoplasm. This wears down when it works, setting free creatin and other nitrogenous débris. Its fuel is glycogen. This is not the way, however, in which the matter is now regarded. Protoplasm is not the machine only, but also the source of power. Glycogen is not burnt in a framework of protoplasm. When muscle contracts, protoplasm casts out CO₂ and H₂O. Glycogen is the food readiest to restore to it the atoms which it has lost.