What is the nature of the material used up in connection with muscle work? As can readily be seen, this is an important question, for on its answer depends, in some measure at least, the character of the proper intake, or food, to be supplied in order to make good the loss. If the energy of mechanical work, the energy of muscle contraction, comes from the breaking down of proteid matter alone, then obviously excessive muscular work would need to be accompanied, or followed, by a generous supply of proteid food. If, on the other hand, external work means liberation of energy solely from non-nitrogenous materials, then it is equally clear that fats and carbohydrates are the proper foods to offset the drain incidental to vigorous muscular action. The views of Liebig, briefly referred to in a previous chapter, held sway over physiologists for many years. His dictum that proteid foods were true plastic foods, entering into the structure of the tissues of the body, and that they alone were the real sources of muscular energy, met for a time with no opposition. It was not until the advent of a more critical spirit, accompanied by a fuller appreciation of the necessity of experimental evidence, that physiologists began to test with scientific accuracy the validity of the current views. It is worthy of note that long prior to this time, even before oxygen was discovered, the far-sighted and resourceful John Mayow, in his work with the various “spirits” of the body and their relation to respiration, etc., evolved the view that muscular power has its origin in the combustion of fat brought to the muscles by the blood and burned there by aid of a gas or “spirit” taken from the air by the lungs, and likewise carried to the muscles by the circulating blood. Considering the time when Mayow lived and the dearth of true scientific knowledge as we measure it to-day, his hypothesis was a wonderful forestalling of present views. It is quite obvious that the views of Liebig, if true, admit of easy proof; since, if the energy of muscular power comes from the breaking down of proteid, there should be a certain parallelism between the output of nitrogen from the body and the amount of muscular work accomplished, everything else being equal. As stated in a previous chapter, such study of this question as was made soon disclosed the fact that the one element above all others that seemed to influence the output of nitrogen was the intake of proteid food. Thus, the English investigators, Lawes and Gilbert, found by experimenting with animals that when the latter were kept under uniform conditions of muscular work, the amount of nitrogen excreted ran parallel with the intake of nitrogen. Further, in the early experiments of Voit, the results obtained clearly showed that variations in the amount of work performed were practically without influence on the excretion of nitrogenous waste products.