There is no question that the energy of muscular contraction can come from all three classes of organic foodstuffs. Voluntary muscular movement is under the control of the nervous system, and when the stimulus is applied the muscle is bound to contract, provided of course there is sufficient energy-containing material present to furnish the means. Muscle tissue, like other tissues and organs, has a certain power of adaptability, by which it is able to do its work, even though it is not adequately supplied with its preferred nutrient. While proteid is plainly not the material from which the energy of muscular contraction is ordinarily derived, it is equally evident that in emergency, as when the usual store of carbohydrate and fat is wanting, proteid can be drawn upon, and in such cases vigorous work may be attended with increased nitrogen output. In harmony with this statement, we find on record in recent years many experiments, both with man and animals, where severe muscular labor is accompanied by an excretion of nitrogen beyond what occurs on days of rest; but by simply adding to the intake of non-nitrogenous food this increased outgo of nitrogen is at once checked. With moderate work, the nitrogen outgo is rarely influenced; it is only when the work becomes excessive, or the store of non-nitrogenous reserve is small and the intake of the latter food is limited, that proteid matter is drawn upon to supply the required energy. Recalling what has been said regarding the significance of the respiratory quotient, it is obvious that we have here a means of acquiring information as to the character of the material that is burned up in the body during muscular work. Increased metabolism of carbohydrate will necessarily result in raising the respiratory quotient, and if the latter food material alone is involved the respiratory quotient must naturally approach 1.0. Zuntz, however, has clearly shown that vigorous muscular activity does not materially change the respiratory quotient; except in cases of very severe work, where the oxygen-supply of the muscles is interfered with. Indeed, the muscles may be made to do work sufficient to increase the consumption of oxygen threefold or more, without any change in the respiratory quotient being observed. And as there is frequently no change whatever in the output of nitrogen under these conditions, it follows that the energy of the muscle work must have come from the decomposition of non-nitrogenous material. If carbohydrates alone were involved, the respiratory quotient would obviously undergo change. Since, however, this remains practically stationary, we are led to the conclusion that fat must be involved in large degree, in addition to carbohydrate.