CHEMISTRY OF DIGESTION AND NUTRITION. 299 



their food was entirely -non-nitrogenous, so that the urea eliminated came 

 entirely from the proteid of the body. Nevertheless, when the urine was 

 collected and the urea estimated it was found that the potential energy contained 

 in the proteid destroyed was entirely insufficient to account for the work done. 

 Although later estimates would modify somewhat the actual figures of their 

 calculation, the margin was so great that the experiment has been accepted as 

 showing conclusively that the total energy of muscular work does not come 

 from the oxidation of proteid alone. Later experiments made by Voit upon 

 a dog working in a tread-wheel and upon a man performing work while in the 

 respiratory chamber (p. 283) gave the surprising result that not only may the 

 energy of muscular work be far greater than the potential energy of the proteid 

 simultaneously oxidized, but that the performance of muscular work within 

 certain limits does not affect at all the amount of proteid metabolized in the 

 body, since the output of urea is the same on working-days as during days of 

 rest. Careful experiments by an English physiologist, Parkes, made upon 

 soldiers while resting and after performing long marches showed also that 

 there is no distinct increase in the excretion of urea after muscular exercise. 

 It follows from these experiments that Liebig's theory as to the source of the 

 energy of muscular work is incorrect, and that the increase in the oxidations 

 in the body which undoubtedly occurs during muscular activity must affect 

 only the non-proteid material, that is, the fats and carbohydrates. Quite 

 recently the question has been reopened by experiments made under Pfliiger 

 by Argutinsky. 1 In these experiments the total nitrogen excreted was deter- 

 mined with especial care in the sweat as well as in the urine and the feces. 

 The muscular work done consisted in long walks and mountain-climbs. 

 Argutinsky found that work caused a marked increase in the elimination of 

 nitrogen, the increase extending over a period of three days, and he estimated 

 that the additional proteid metabolized in consequence of the work was suf- 

 ficient to account for most of the energy expended in performing the walks 

 and climbs. A number of objections have been made to Argutinsky's work. 

 It has been asserted that during his experiment he kept himself upon a 

 diet deficient in non-proteid material ; that if the supply of this material had 

 been sufficient, none of the additional proteid would have been oxidized. It 

 must be admitted, however, that the experiments of Argutinsky compel us to 

 state the proposition above as to the relation between muscular work and 

 proteid metabolism in a more careful way. It is necessary to modify the 

 statement generally made to the extent of saying that muscular work causes 

 no increase in proteid metabolism, provided the supply of food is abundant. 

 If now we compare the amounts of CO 2 eliminated during work and during 

 rest, it will be found that there is a very decided increase during work. In the 

 experiments made by Pettenkofer and Voit the CO 2 given off by a man 

 during a day of muscular work was nearly double that eliminated during a 

 resting-day. Indeed, the same fact has been observed repeatedly upon isolated 

 muscles made to contract by artificial stimuli. Assuming, then, that muscular 

 1 Pftilger's Archiv fiir die gesammte Physiologic, 1890, vol. 46, p. 552. 



