538 A MANUAL OF PHYSIOLOGY 



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 circumstance, and full of instruction for 

 the ingenuous student of science, that the facts which have been 

 supposed absolutely to disprove the older theory, and absolutely 

 to establish its modern rival, 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 has again been made too absolute. The pendulum 

 must again swing back a little ; and the experiments of Pfliiger 

 and others have actually 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, severe 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 third ; and on the first and second rest days, 

 following work, the elimination 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. 



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 case the excretion of nitrogen 

 would not necessarily be altered ; the proteins which, in the 

 absence of work, would have been oxidized within the muscular 

 substance or elsewhere, their energy appearing entirely as heat, 

 may, when the call for protein to take the place of that broken 

 down in muscular contraction arises, be diverted to this 

 purpose. 



In any case, there is no doubt that a dog fed on lean meat may 

 go on for a long time performing far more work than can be 



