GENERAL METHODS— HISTORY OF PROTEIN FOOD. 891 



capacity of the body to digest and absorb protein food, nitrogen 

 equilibrium may be maintained upon any given amount of protein. 



Carbon Equilibrium and Body Equilibrium.— The term "car- 

 bon equilibrium" is sometimes used to describe the condition in 

 which the total carbon of the excreta (in the carbon dioxid, urea, 

 etc.) is balanced by the carbon of the food. It is possible that an 

 individual may be in nitrogen equilibrium and yet be losing or gain- 

 ing in weight, since, although the consumption of proteins may just 

 be covered by the proteins of the food, so far as the nitrogen is con- 

 cerned, the supply of food may be greater than the body needs or 

 can destroy, and the excess may be stored in the body in the form of 

 glycogen or fat, or, on the other hand, the supply of energy-yielding 

 food may be less than is necessary to cover the needs of the body 

 and some of the fat of the body may be oxidized. In the first case 

 the carbon absorption would exceed the carbon excretion — there 

 would be a plus carbon balance; in the second case the reverse 

 would hold. In the same way we may speak of a water equilibrium 

 or a salts equilibrium, although these terms are not generally used. 

 An adult under normal conditions lives so as to maintain a gen- 

 eral body equilibrium ; his ingesta of all kinds are balanced by the 

 corresponding excretions, and the individual maintains a practi- 

 cally constant body-weight. 



Complete Balance Experiments — Respiration Chamber. — 

 According to the statements made in the last paragraph, it is obvious 

 that if the analytical work is properly done, an exact balance may 

 be drawn between the proteins, fats, and carbohydrates eaten as 

 food and the proteins, fats, and carbohydrates destroyed in the 

 body as represented by the nitrogen and carbon contained in the 

 excreta. Complete experiments of this kind were attempted first 

 by Voit * and Pettenkofer, to whose work much of our fundamental 

 knowledge is due. In the experiments of these authors, made upon 

 men as well as animals, the total nitrogen of the urine and feces was 

 determined and the total quantity of CO. given off from the lungs 

 was estimated. This last determination was made possible by 

 placing the individual in a specially constructed chamber or respi- 

 ration apparatus. Air was drawn through this room by means of 

 a pump. The total quantity of air passing through the room was 

 measured by a gasometer and definite fractions were drawn off 

 from time to time, which were analyzed for COj. From the 

 figures thus obtained it was possible to estimate the entire CO2 

 given off during the period of observation. Knowing the total 

 nitrogen and carbon eliminated, it was possible to estimate the 

 amount of protein and fat or carbohydrate destroyed in the body. 

 From the nitrogen the quantity of protein metabolized was 

 * See Hermann's " Handbuch der Physiologic, " vol. vi., 1881. 



