TOPICS: Relation of muscle work to energy exchange. Views of Liebig. Experimental evidence. Relation of nitrogen excretion to muscle work. Significance of the respiratory quotient in determining nature of the material oxidized. Fats and carbohydrates as source of energy by muscles. Utilization of proteid as a source of energy. Formation of carbohydrate from proteid. Significance of proteid metabolism. Theories of Carl Voit. Morphotic proteid. Circulating proteid. General conception of proteid metabolism on the basis of Voit’s theories. Pflüger’s views of proteid metabolism. Rapidity of elimination of food nitrogen. Methods by which nitrogen is split off from proteid. Theories of Folin. Significance of creatinin and of the percentage distribution of excreted nitrogen. Endogenous or tissue metabolism. Exogenous or intermediate metabolism. Needs of the body for proteid food possibly satisfied by quantity sufficient to meet the demands of tissue or endogenous metabolism. Bearings of Folin’s views on current theories and general facts of proteid metabolism. Large proteid reserve and voluminous exogenous metabolism probably not needed. Importance of feeding experiments in determining the true value of different views. As we have already seen, every form of muscular activity begets an increase in the energy exchange of the body. Between the two extremes of absolute rest and excessive muscular exertion, we find differences of 2000 calories or more per day as representing the degree of chemical decomposition corresponding to the particular state of muscular activity. The work of the involuntary muscles, such as have to do with peristalsis, respiration, rhythmical beat of the heart, etc., is a relatively constant factor, though naturally subject to some variation, as has been pointed out in other connections. External muscular activity, however, is the one factor above all others that modifies the rate of energy exchange. A little longer walk, a heavier load to carry, a steeper hill to climb, any increase great or small in the activity of the muscles of the body, means a corresponding increase in chemical decomposition, with increased output of the ordinary products of tissue oxidation. The material so consumed, or oxidized, must be made good to hold the body in equilibrium; the supplies drawn upon are to be replaced, if the tissues of the body are to be kept in a proper state of efficiency.