CHEMISTRY OF DIGESTION AND NUTRITION. 283 



proteid. By this means all the nitrogenous excreta which may occur in the 

 urine are allowed for ; and since the various proteids differ but little in the 

 amount of nitrogen which they contain, the average being from 15.5 to 16 per 

 cent., it is only necessary to multiply the total quantity of nitrogen found in the 

 excretions by 6.25 (proteid molecule : N : : 100 : 16) to ascertain the amount of 

 proteid destroyed. In accurate calculations it is necessary to determine the total 

 nitrogen in the feces as well as in the urine, and for two reasons : first, in ordi- 

 nary diets a certain proportion of vegetable and animal proteid escapes digestion, 

 and this amount must be determined and deducted from the total proteid eaten 

 in order to ascertain what nitrogenous material has actually been taken into the 

 body ; second, the secretions of the alimentary canal contain a certain quan- 

 tity of nitrogenous material, which represents a genuine excretion, and should 

 be included in estimates of the total proteid-destruction. Practical experience 

 has shown that in man about 29 per cent, of the total nitrogen of the feces has 

 this latter origin. The nitrogen eliminated as urea, etc. in the sweat, milk, 

 and saliva is neglected under ordinary circumstances because the amount is 

 too small to affect materially any calculations made. To determine the total 

 amount of non-nitrogenous material destroyed in the body during a given 

 period, two data are required : first, the total nitrogen in the excreta of the 

 body ; second, the total amount of carbon given off from the lungs and in the 

 various excreta. From the total nitrogen one calculates how much proteid was 

 destroyed, and, deducting from the total carbon the amount corresponding to 

 this quantity of proteid, what remains represents the carbon derived from the 

 metabolism of the non-nitrogenous material that is, from the fat or carbo- 

 hydrate. By methods of this kind it is possible to reckon back from the 

 excreta to the total amount of material, consisting of proteid, fat, and carbo- 

 hydrate, which has been consumed in the body within a certain period. If, now, 

 by analyzing the food or by making use of analyses already made (see p. 216), one 

 determines how great a quantity of proteid, fat, and carbohydrate has been taken 

 into the body in the same period, then, by comparison of the total ingesta and 

 egesta, it is possible to strike a balance and to determine whether all the proteid, 

 fat, and carbohydrate of the food have been destroyed, or whether some of the 

 food has been stored in the body, and in this case whether it is nitrogenous or 

 non-nitrogenous material, or, lastly, whether some of the reserve material of 

 the body, nitrogenous or non-nitrogenous, has been destroyed in addition to 

 the supply of food. It is needless to remark that " balance experiments " of 

 this character are very laborious, particularly as they must be made over long 

 intervals one or more days. Nevertheless, a great deal of work of this 

 kind has been done upon man as well as upon lower animals, especially by 

 Voit 1 and Pettenkofer. In the experiments upon man the urine and feces 

 were collected carefully and the total nitrogen was determined ; at the same time 

 the total quantity of CO 2 given off from the lungs was estimated for the entire 

 period. The determination of the CO 2 was made possible by keeping the man 

 in a specially-constructed chamber through which air was drawn by means of a 

 1 Hermann's Handbuch der Physiologie, 1881, vol. vi. 



