NUTRITION 355 



the nitrogen has not been lost. It has passed through the 

 system, and performed some function we cannot explain, and 

 finally, from an agricultural point of view, it has conferred on 

 manure its only value. Nevertheless, some diets are wastefully 

 nitrogenous, and exact experiments on men have shown that 

 they can be kept in health for months on a diet far poorer in 

 protein than that generally accepted as necessary, and the 

 same finding, within limits, holds good for animals. On the 

 matter of food nitrogen many physiologists come into conflict 

 with practical experience. 



Theory says the quantity of nitrogen required is largely inde- 

 pendent of muscular work ; practice says the harder the machine 

 is worked the more nitrogen must be given. Theory says 

 proteins are not the source of muscular energy, this being the 

 function of non-nitrogenous food ; practice replies that may be 

 so, but experience shows that the harder the work performed 

 by an animal, the more strongly nitrogenous must the diet be, 

 while the amount of the latter is only to be limited by the 

 appetite. In this matter our personal experience places us 

 entirely on the side of practice and opposed to theory. Why the 

 hard-worked horse needs more nitrogen we are not prepared to 

 explain. The suggestion that the machine works more easily 

 and smoothly on a liberal nitrogenous diet, which stimulates 

 metabolism, and so leads to increased oxidation, does not bring 

 us much nearer to a solution of the problem. The fact remains 

 that whatever may be the energy obtainable from starch and fat, 

 this energy is in some unknown way directed by protein. All 

 nitrogen over and above that required for repair is considered a 

 wasteful or luxus consumption — a condition to which we by 

 no means subscribe. That a wasteful consumption of protein 

 occurs where horses are not fed in accordance with the work they 

 are performing is undoubted. Under these circumstances the 

 excess of nitrogenous material produces clinical effects ; we are 

 able to recognise them in the liver disorders and diarrhoea of 

 tropical climates, and lymphangitis, azoturia, and diarrhoea of 

 temperate latitudes. 



Doubtful and difficult of solution as many of the important 

 points are in nitrogenous feeding, they are nothing in com- 

 parison with the problem of how the dead food-protein is con- 

 verted into the living body-protein, and how the same kind of 

 protein can be utilised in building up material so different in 

 structure as bone and brain, muscle and fat, liver and skin. 



The storing up of protein occurs, as we have seen, in young 

 animals, and in working animals so long as the muscles are 

 increasing in bulk. In making this statement it is necessary to 

 remember that it does not exclude the daily repair of the tissues. 



