73 o THE POPULAR SCIENCE MONTHLY. 



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 im- 

 mediate 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 organ- 

 ism, 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 pro-' 

 duction 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 follow- 

 ing. They found that the amount of nitrogen eliminated by the kid- 

 neys 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 



