METABOLISM, NUTRITION AND DIETETICS 537 



supposed to be destroyed ; the building remains, for although stones 

 are constantly crumbling in its walls, others are being constantly 

 built in. 



A much less plausible view is that the tissue elements themselves are 

 short-lived ; that the old cells disappear bodily and are replaced by 

 new cells ; and that the whole of the proteins of the food take part 

 in this process of total ruin and reconstruction. Histological 

 evidence is strongly against this idea. Although the cells of certain 

 glands, such as the mammary, and perhaps the mucous glands, 

 exhibit changes which, hastily interpreted, might seem to indicate 

 that, like those of the sebaceous glands (p. 473), they break down 

 bodily, as an incident of functional activity, there is no proof of 

 the production of new cells on the immense scale which this theory 

 would require. 



A second law of nitrogenous metabolism is that within normal 

 limits it is nearly independent of muscular 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, pro- 

 teins 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 Fick and Wislicenus in climbing a mountain) 

 on a non-nitrogenous diet had double the heat equivalent of 

 the whole of the protein 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 pro- 

 tein broken down was eliminated during the time of this experi- 

 ment, a part at least of the work must have been derived from 

 the energy of non-nitrogenous material. And the increase in 

 the carbon dioxide given off, which is as conspicuous an accom- 

 paniment of muscular work as the constancy of the urea excre- 

 tion, showed that during muscular exertion carbonaceous 

 substances other than proteins that is to say, fats and carbo- 

 hydrates are oxidized in greater amount than during rest. 



So the pendulum of physiological orthodoxy came full-swing 

 to the other side. Liebig and his school had taught that pro- 

 teins alone were consumed in functional activity ; the majority 

 of later physiologists have denied to the proteins any share 

 whatever in the energy which appears as muscular contraction. 

 The proteins, they say, ' repair the slow waste of the frame- 

 work of the muscular machine, replace a loose rivet, a worn-out 

 belt, as occasion may require ; the carbo-hydrates and fats are 



