METABOLISM, NUTRITION AND DIETETICS 469 



must again swing back a little ; and the recent 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 ; exces- 

 sive 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 xanthin 

 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 proteids. For the 

 animal body is a beautifully-balanced mechanism which 

 constantly adapts itself to its conditions. If it saves 

 proteids by the use of fat and carbo-hydrates when its 

 nitrogenous food is restricted or its organ-proteid runs low, 

 it may also, when called upon to labour, save proteids from 

 lower uses to devote them to muscular contraction. In this 

 case the excretion of nitrogen would not necessarily be 

 altered ; the proteids which, in the absence of work, would 

 have been oxidized within the muscular substance or else- 

 where, their energy appearing entirely as heat, may, when 

 the call for proteid to take the place of that broken down 

 in muscular contraction arises, be diverted to this purpose. 



Thirdly, 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 yielded by the energy of fat and carbo-hydrates 

 occurring in traces in the food, or taken from the stock in 

 the animal's body at the beginning of the period of work. 

 A large portion, and perhaps the whole, of the work, must 

 in this case be derived from the energy of the proteids 

 (Pfliiger). On the other hand, it is well established that 

 when fats and carbo-hydrates are present in sufficient 



