CHEMISTRY OF DIGESTION AND NUTRITION. 345 



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 C0 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 mi 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 

 due to a loss or a gain in body-fat — in other words, to changes in the carbon 

 equilibrium. 



Nutritive Importance of the Proteids. — The digestion and absorp- 

 tion of proteids have been considered in previous sections. We believe that 

 the digested proteid, with the exception of the variable quantity that suffers 

 decomposition in the intestine as a result of putrefaction or of the prolonged 

 action of trypsin, is absorbed into the blood after undergoing an unknown 

 modification during the act of absorption. Subsequently tins proteid 

 materia] passes into the lymph ami is broughl into contacl with the 

 tissues. lt> main nutritive importance lies in its relations to the tissues, 

 and, speaking generally, we may say that the final late of the proteid 

 molecule is that it undergoes a physiological oxidation whereby the complex 

 molecule is broken down to form the simpler and more stable compounds, 

 C0 2 , H 2 0, urea, sulphates and phosphates. This destruction of the proteid 

 molecule takes place in or under the influence of the living cells, and it 

 gives rise to a liberation of energy mainly in the form of heat. It is 

 impossible to follow the various ways in which this physiological oxidation 

 takes place. It is probable, however, that some of the proteid undergoes 

 destruction without becoming a part, an organized part, of the living cells, 

 although its oxidation is effected through the agency of the cells. It has been 

 proposed by Yoit ' to designate the proteid that is oxidized in this way as 

 1 Hermann's Handbuch der Physiobgie, 1881, Bd. vi. S. 300. 



