6io METABOLISM, NUTRITION AND DIETETICS 



(p. 588). Some writers conceive that in such a short-cut from pro- 

 tein to urea we have a kind of physiological safety-valve to protect 

 the tissues from the burden of an excessive metabolism. And il 

 by this is meant that it is advantageous to the tissues that a special 

 mechanism should exist to eliminate a surplus of nitrogen which they 

 do not require, and which they cannot store, and to present them 

 with a residue which they can utilize, the conception is certainly 

 correct. But there is no good evidence that in the presence of an 

 over-abundant supply of amino-acids the endogenous protein meta- 

 bolism would be essentially modified. 



Relation of Nitrogenous Metabolism to Muscular Work. This is 

 another of those classical physiological problems which it is difficult 

 to present properly apart from its historical setting. The general 

 result of much experimental work and long-continued discussion is 

 that when the work does not transgress what may be called ' normal 

 limits,' the excretion of nitrogen is nearly independent of mus- 

 cular work that is to say, the quantity of nitrogen excreted by a 

 man on a given diet is practically the same whether he rests or works. 

 Before this was known it was maintained by Liebig that proteins 

 alone could supply the energy of muscular contraction that, in 

 fact, proteins were solely used up in the nutrition and functional 

 activity of the nitrogenous tissues, while the non-protein food 

 yielded heat by its oxidation. As exact experiments multiplied, it 

 was found that muscular work, the production of which is the 

 function of by far the greatest mass of protein-containing tissue, 

 had little or no effect upon the excretion of urea in the urine. More 

 than this, it was shown that a certain amount of work accomplished 

 (by Pick and Wislicenus in climbing a mountain) on a non-nitrog- 

 enous diet had double the heat equivalent of the whole of the pro- 

 tein consumed in the body, as estimated by the urea excreted during, 

 and for a given time after, the work. On the assumption that all 

 the urea corresponding to the protein broken down was eliminated 

 during the time of this experiment, a part at least of the work must 

 have been derived from the energy of non-nitrogenous material. 

 And other experiments in which account was taken of the increase 

 in the carbon dioxide given off (as conspicuous an accompaniment 

 of muscular work as the constancy of the urea excretion), showed 

 that during muscular exertion carbonaceous substances other than 

 proteins that is to say, fats and carbo-hydrates are oxidized in 

 gi eater amount than during rest. 



So the pendulum of physiological orthodoxy came full-swing to the 

 other side. Liebig and his school had taught that proteins alone were 

 consumed in functional activity; the majority of later physiologists 

 following Voit denied to the proteins any share whatever in the energy 

 which appears as muscular contraction. The proteins, they said, 

 1 repair the slow waste of the framework of the muscular machine, 

 replace a loose rivet, a worn-out belt, as occasion may require ; the 



