300 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



work causes no increase in the nitrogen excreted, but a marked increase in the 

 CO 2 eliminated, we are justified in saying that the energy of muscular work 

 under normal conditions comes mainly, if not exclusively, from the oxidation 

 of non-proteid material. The machine that does the work, the muscle, is 

 par excellence a proteid tissue, but the normal resting metabolism of its pro- 

 teid substance is not increased by the chemical changes of contraction. Or, 

 to put it in another way, the chemical changes which give rise to the energy 

 liberated in contraction involve only the non-proteid material. It is interest- 

 ing to remember in this connection that the consumption of glycogen, or of 

 the sugar derived from it, is intimately connected with muscular work. The 

 glycogen of the body in an animal deprived of food disappears much more 

 rapidly if the animal is made to work his muscles than if he remains at 

 rest. In an experiment by Kiilz upon well-fed dogs it was found that the 

 glycogen was practically all used up in a single fasting-day during which the 

 animals did a great deal of work. Morat and Dufourt have shown also that 

 a muscle after prolonged contraction takes much more sugar from the blood than 

 it did previous to the contraction, and Harley * finds that power to perform 

 muscular work may be increased and susceptibility to fatigue be diminished 

 by eating sugar in quantities. It is, in fact, generally agreed that glycogen is 

 used up in muscle-contractions, but the way in which the destruction of the 

 glycogen is effected is not definitely known. After the glycogen has been con- 

 sumed it is probable that the other constituents of the body, the fats and the 

 proteids, are called upon to furnish the necessary energy. For this reason 

 we should expect, in a person performing excessive muscular work, that there 

 would be an increased destruction of proteid when the supply of non-proteid 

 food is insufficient. 



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 proteid metabolism; on the contrary, the CO 2 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 metab- 

 olism. 



Effect of Variations in Temperature. In warm-blooded animals variations 

 of outside temperature within ordinary limits do not affect the body-tem- 

 perature. A full account of the means by which this regulation is effected 

 will be found in the section upon Animal Heat. So long as the temper- 

 ature of the body remains constant, it has been found that a fall of outside 

 temperature increases the oxidation of non-proteid material in the body, the 

 increase being in a general way proportional to the fall in temperature. That 

 the increased oxidation affects the non-proteid constituents is shown by the 

 fact that the urea remains unchanged in quantity, other conditions being the 

 same, while the oxygen-consumption and the CO 2 -elimination are increased. A 

 1 Journal of Physiology, 1894, vol. xvi. p. 97. 



