842 NUTRITION AND HEAT REGULATION. 



the non-proteins are not sufficient in quantity some of the energy 

 is obtained at the expense of the protein of the body, and there is 

 an increase in the nitrogen excretion. We may believe, in fact, 

 that the energy necessary for muscular work may be obtained from 

 any of the heat-yielding foodstuff carbohydrates, fat, or proteins. 

 It seems probable that the sugar (glycogen) of the muscle is, so to 

 speak, the easiest source; but, when the carbohydrates are deficient 

 or absent altogether in the diet, muscular exercise is accompanied 

 by an increase in the consumption of proteins or fats or both. The 

 Voit theory is correct to the extent that on an abundant non-protein 

 diet much muscular work may be done without any increase in the 

 consumption of protein tissue. The muscle is a protein machine 

 for the accomplishment of work, and the fuel supplied may be pro- 

 tein or non-protein, but in the accomplishment of moderate work 

 there is apparently no greater wear and tear of the machinery, no 

 greater tissue waste, than under resting conditions. If, however, 

 the muscular work is excessive, the tissue waste may be increased. 

 Argutinsky found an increased nitrogen elimination lasting two or 

 three days after the cessation of the work. It is probable that this 

 result indicates a greater waste of the protein apparatus itself, and 

 this idea is borne out by the fact that under similar conditions 

 other observers have detected an increase in the creatinin and 

 uric acid excretion, nitrogenous wastes that are derived from 

 muscle. The effect of muscular work on the carbon excretion, car- 

 bon dioxid, is, of course, marked and invariable. Some extra ma- 

 terial must be oxidized to furnish the energy, and since this material 

 is usually sugar, or sugar and fat, or the non-nitrogenous portion 

 of the protein of the diet, the effect, so far as the excretions are con- 

 cerned, will be most manifest in the amount of carbon dioxid 

 given off. Pettenkofer and Voit found that the carbon dioxid 

 eliminated by a man during a day of work was nearly double that 

 excreted during a day of rest. Along with this rise in the carbon 

 dioxid excretion there is a corresponding increase in the absorption 

 of oxygen. 



Metabolism during Sleep. It has been shown that during 

 sleep there is no marked diminution of the nitrogen excreted, and 

 therefore no distinct decrease in the protein metabolism; on the 

 contrary, the carbon dioxid eliminated and the oxygen absorbed 

 are unquestionably diminished. This latter fact finds its simplest 

 explanation in the supposition that the muscles are less active 

 during sleep. The muscles do less work in the way of contractions, 

 and, in addition, probably suffer a diminution in tonicity, which 

 also affects their total metabolism. 



Effect of Variations in Temperature. In warm-blooded 

 animals variations of outside temperature within ordinary limits 



