341 .4.V AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



tliis quantity of proteid, what remains represents the carboD 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 lias 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. 278), 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 ' 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 C0 2 given off from the lungs was estimated for the entire 

 period. The determination of the C0 2 was made possible by keeping the man 

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

 pump; the total quantity of air drawn through was indicated by a gasometer, 

 and a measured portion of this air was drawn off through a separate gasometer 

 and was analyzed for its C0 2 . It was found that the method is practicable : that 

 by the means described a nearly perfect balance may be struck between the income 

 and the outgo of the body. Experiments of this general character have been 

 used to determine the fate of the food-stuffs in the body under different con- 

 ditions, the essential part that each food-stuff takes in general nutrition, and 

 so on. In this and the succeeding sections we shall have to consider some of 

 the main results obtained ; but first it will be convenient to define two terms 

 frequently used in this connection — namely, " nitrogen equilibrium " and 

 " carbon equilibrium." 



Nitrogen Equilibrium. — By "nitrogen equilibrium" we mean that condition 

 of an animal in which, within a definite period, the nitrogen of the excreta is 

 equal in amount to the nitrogen of the food ; in other words, that condition 

 in which the proteid (and albuminoid) food eaten exactly covers the loss of 

 proteid (and albuminoid) in the body during the same time. W an animal 

 is giving off more nitrogen in its excreta than it receives in its food, then 

 the animal must be losing proteid from its body; if, on the contrary, the food 

 that it eats contains more nitrogen than is found in the excreta, the animal must 

 oe storing proteid in its body. A condition of nitrogen equilibrium is the 

 normal state of a properly-nourished adult. It is important to remember that 

 nitrogen equilibrium may be maintained at different levels; that is, one may 

 ' Hermann'* Handiuch der Physwlogie, 18S1, lid. vi. 



