METABOLISM OF MATTER AND ENERGY. 1011 



excreted ill the the urine, and, futherinore, it is excreted within a com- 

 paratively short time. The nitrogen in the food minus the nitrogen 

 in the feces (which is ordinarily assumed to represent the digested 

 nitrogen) is very nearly the same as the amount excreted in the urine. 

 If the nitrogen of metabolic products in the feces is determined and 

 added to the amount in the urine it is probable that, provided the sub- 

 ject is in nitrogen equilibrium, this sum would more truly represent 

 the total amount of digested nitrogen than the values which are 

 obtained by ordinary methods. At any rate, the values so obtained 

 are useful as a check on the results obtained by taking the ditierenee 

 between food and feces as the amount digested. 



In digestion experiments with animals little account is taken of the 

 fact that the digestibility of a food is influenced by the foods which 

 are consumed with it, In experiments with men on an absolute milk 

 diet, a considerable quantity of nitrogen may be excreted in the feces, 

 owing probably to the fact that the milk when consumed in quantity 

 coagulates in the stomach and the masses are not sufficiently broken 

 up in their passage through the intestinal tract so that the digestive 

 juices can act upon the whole mass. On the other hand, if bread is 

 consumed with the milk the particles of bread prevent the formation 

 of large masses of casein in the stomach and the material is in better 

 mechanical condition for digestion, and is actually more thoroughly 

 digested; that is, consuming bread with milk increases the digestibility 

 of the milk. Furthermore, in such experiments it is often impracti- 

 cable to continue a ration consisting of a single food material for any 

 considerable length of time, because unusual conditions may readily be 

 supposed to eventually produce abnormal results. In all such cases 

 a (heck on the accuracy of the results is even more valuable than in 

 digestion experiments under normal or usual conditions. 



As in the case of feeding experiments, determining the nitrogen bal- 

 ance is useful in showing the regularity of progress of digestion experi- 

 ments, since if the nitrogen balance does not vary from day to day 

 under like experimental conditions it may be assumed that the experi- 

 mental conditions are under control, and the results of one investiga- 

 tor may be more readily compared with those of another. 



Other experiments in which the iiitr<></<-u balance is of use. — There are 

 many other matters connected with the subject of nutrition besides 

 feeding and digestion experiments which need investigation, and are in 

 line with the work followed at many of the stations, in which a determi- 

 nation of the balance of income and outgo of nitrogen is either essen- 

 tial or useful. A proper understanding of the functions of food and 

 the amount of food requisite for different purposes, including the pro- 

 duction of work, necessitates a knowledge of the source of energy in 

 the animal body. While it is recognized that energy is supplied by 

 the food, there are many theories as to which of the nutrients is the 

 real source of energy in the body. It is now generally held that protein, 

 18766— No. 11 2 



