284 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 CO 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 

 u 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. If 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 

 be storing proteid in its body. The 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 

 begin with a starving animal and slowly increase the amount of nitrogenous food 

 until nitrogen equilibrium is just established. If now the amount of nitrog- 

 enous food is increased say doubled the excess does not, of course, continue 

 to be stored up in the animal's body ; on the contrary, in a short time the 

 amount of proteid destroyed in the body will be increased to such an extent 

 that nitrogen equilibrium will again be established at a higher level, the animal 

 in this case eating more and destroying more. The highest limit at which nitro- 

 gen equilibrium can be maintained is determined, apparently, by the power 

 of the stomach and the intestines to digest and absorb proteid food. Further 

 details upon this point will be given presently, in describing the nutritive 

 value of the food-stuffs. 



Carbon Equilibrium. The term " carbon equilibrium " is sometimes used 

 to describe the condition in which the total carbon of the excreta (occurring in 

 the CO 2 , urea, etc.) is exactly covered by the carbon of the food. As one can 

 readily understand, an animal might be in a condition of nitrogen equilibrium 

 and yet be losing or be gaining in weight, since, although the consumption of 

 proteids in the body might just be covered by the proteids of the food, the 

 consumption of non-proteids, fats and glycogen, might be greater or less than 

 was covered by the supply of food. In addition, we might speak of -an equi- 

 librium as regards the water, salts, etc., although these terms are not generally 

 used. An adult in good health usually so lives as to keep in both nitrogen 

 and general body equilibrium that is, to maintain his normal weight while 

 slight variations in weight from time to time are probably for the most part 



