GENERAL METHODS HISTORY OF PROTEID FOOD. 785 



the body to digest and absorb proteid food nitrogen equilibrium 

 may be maintained upon any given amount of proteid. 



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 gaining in weight, 

 since, although the consumption of proteids may just be covered 

 by the proteids of the food, the consumption of non-proteid material, 

 particularly the fats of the body, may be greater than the supply 

 furnished by or manufactured from the food. An animal may 

 lose or gain in carbon when his nitrogen supply is in equilibrium. 

 In the same way under special circumstances 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 general body equilibrium; his ingesta of all kinds 

 are balanced by the corresponding excretions, and the individual 

 maintains a practically constant body-weight. 



Complete Balance Experiments Respiration Chamber. Ac- 

 cording 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 proteids, fats, and carbohydrates eaten as 

 food and the proteids, 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 2 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 for analysis of its CO 2 . From the figures thus 

 obtained it was possible to estimate the entire CO 2 given off during 

 the period of observation. Knowing the total nitrogen and CO 2 

 eliminated, it is possible to estimate the amount of proteid and 

 fat or carbohydrate destroyed in the body. If the carbon belong- 

 ing to the amount of proteid metabolized is deducted from the total 

 carbon excreta what is left represents either fat or carbohydrate 

 burnt in the body, and, knowing the amount of these materials 

 taken in the diet, it is possible to ascertain whether the correspond- 

 ing amount of carbon has. all been excreted. By experiments of 



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

 50 



