certain amount of force is exerted in the processes of circulation of the blood and in the respiratory movements, and a certain amount of work is performed by muscular action. If it be assumed that the oxidation of matter in the animal economy involves, of necessity, either the production of heat or of force, an answer to the following question becomes at once of great importance as regards our ideas of the immediate source of muscular power:
Is the food directly oxidized in the perfected and adult animal organism, the result of this oxidation being heat and force, and is this the single source of muscular power, or is the perfected animal organism, particularly as regards the muscular system, itself consumed gradually as a result of muscular work, the waste of muscular tissue being represented by the excretions, and such waste being repaired constantly by food? To state this question in simpler terms, is the muscular system a part of a machine that consumes food as fuel in the production of force, not wearing its own substance to any considerable extent, or does the muscular system use its own substance in the production of force?
Before 1866, the following ideas, formulated by Liebig, Lehmann, and others, were pretty generally accepted by physiologists:
The muscular substance, which constitutes about two-fifths of the weight of the entire body, is composed mainly of matters containing carbon, hydrogen, oxygen, and nitrogen, in contradistinction to the fats, which contain only carbon, hydrogen, and oxygen. The most important excrementitious matter thrown off by the kidneys is urea, which contains carbon, hydrogen, oxygen, and nitrogen. The amount of urea excreted is to be regarded, to a certain extent, as a measure of the physiological wear of the muscular system, which wear is increased by muscular exertion, there being a corresponding increase in the excretion of urea by the kidneys. This wear of the muscular system is being constantly repaired by the nitrogenized elements of food. In discussing, then, this question, physiologists have come to speak of the excretion of nitrogen as measuring, more or less accurately, the physiological wear of the muscular system.
In 1866, two German physiologists, Fick and Wislicenus, ascended one of the Alpine peaks, and measured the influence of this unusual muscular exertion upon the excretion of nitrogen. As it is well known that the quantity of nitrogenized food, such as meat, eggs, etc., influences the amount of nitrogen excreted by the kidneys, these observers confined themselves to a diet without nitrogen during the ascent and for a number of hours immediately preceding and following. They found that the amount of nitrogen eliminated by the kidneys was diminished during the muscular exertion by about one-third. From these experiments, they concluded that muscular exercise does not increase the elimination of nitrogen, but rather diminishes it; and from this time dates the proposition, which is now adopted by many
