600 METABOLISM. NUTRITION AND DIETETICS 



carbo-hydrates and fats are the fuel which feeds the furnaces of life, 

 the material which, dead itself, is oxidized in the interstices of the 

 living substance, and yields the energy for its work.' 



Now, it is a singular coincidence, and full of instruction for the 

 ingenuous student of science, that the facts which were supposed 

 absolutely to disprove the older theory, and absolutely to establish its 

 more modern rival, are now seen to do neither the one thing nor the 

 other. The fact and it is a fact that the excretion of nitrogen is 

 but little affected by muscular contraction, does not prove that none 

 of the energy of muscular work comes from proteins; the fact that, 

 under certain conditions, some of the muscular energy must apparently 

 come from non-nitrogenous materials, does not prove that these are the 

 normal source of it all. The distinction had again been made too 

 absolute. The pendulum must again swing back a little; and the 

 experiments of Pfliiger and his pupils were soon to set it moving. 



In the first place, it is not perfectly correct to say that work 

 causes no increase in the excretion of nitrogen; excessive work in 

 man, and work, seveje but not excessive, in a flesh- fed dog (Pfliiger), 

 do cause somewhat more nitrogen to be given off. On the first day 

 of work the increase is always much less than on the second and t rrrd ; 

 and on the first and second rest days, following work, the elimina- 

 tion of nitrogen is still increased. After excessive exercise in man 

 not only is the urea increased, but also the ammonia, kreatinin, and 

 if the subject is in poor training, the uric acid and purin bases (Paton, 

 Stockman, etc.). Moderate exercise causes no increase on the first 

 day, but a slight increase on the second. The meaning of these 

 facts seems to be that during muscular work the intensity of which 

 does not exceed certain limits, the protein waste of the muscular 

 substance itself is no greater than during rest. When, however, the 

 machine is * speeded up ' beyond a certain point the wear and tear 

 is sensibly increased and an excess of tissue-protein is katabolized. 

 There is no reason to suppose that the tissue-protein thus broken 

 down will not yield energy for the muscular work by the oxidation 

 of its non-nitrogenous residue, just as well as the surplus amino- 

 bodies derived from food-protein. The muscular machine has the 

 peculiarity that it is constructed of combustible material; even the 

 dust and the splinters, if we may so express it, which represent the 

 wear and tear of the machine can be burnt in the furnace which keeps 

 it going. 



In the second place, even if the excretion of nitrogen were entirely 

 unaffected by work, this would not prove that none of the 

 energy of the work comes from proteins. For, as we have seen, 

 it is after the nitrogen has been split off and converted into urea 

 that the energy of a great part of the food-protein is developed by 

 oxidation. Further, since the animal body is a beautifully-balanced 

 mechanism which constantly adapts itself to its conditions, it is 

 conceivable that it may, when called upon to labour, save proteins 

 from lower uses to devote them to muscular contraction. In this 



