METABOLISM. 489 



frame by the aid of simple elements. The plant is a synthetic labora- 

 tory of chemistry. But this is not true of the animal organization. 

 The latter is incapable of anabolism and life except by the aid of 

 complex food-combinations such as have been formed by the plant. 

 Contrary to the plant, the animal is a laboratory of analytical 

 chemistry. The animal can only form by synthesis combinations of a 

 low degree, as water, benzoic acid and ammonia, which cannot be built 

 up in the animal. But the plant can take H, 0, C0 2 and N, and from 

 them make complex and elevated combinations. 



Balance of Nutritive Exchange. 



To ascertain the balance of nutritive exchange, a comparison is 

 made between the ingesta and egesta: between the gains and losses. 

 The ingesta consist of food and oxygen; the egesta of various excreta 

 and of the carbon dioxide and water lost by the lungs and skin. When 

 the ingesta equal the egesta and the organism neither gains nor loses 

 weight, there is a complete nutritive equilibrium. 



A balance of water is made by giving, upon the one side, the 

 quantity of water ingested by the foods and drinks; upon the other, 

 the quantity of water eliminated by the stools, urine, skin and lungs. 

 As the hydrogen contained in the food is oxidized and transformed 

 into water, it is evident that in a state of equilibrium the quantity of 

 water eliminated will be much greater than that ingested. By com- 

 paring the water ingested with the water egested, it is found how 

 much oxygen serves to burn the hydrogen. 



Definite enough information is obtained regarding the balance 

 of metabolism if the nitrogen and carbon only are determined in the 

 ingesta and egesta. 



The balance of proteid is made by a comparison of the nitro- 

 gen ingested with that egested, for the amount of nitrogen permits us 

 to know the quantity of proteid, since 100 parts of proteid contain 16 

 parts of nitrogen. The nitrogen eliminated is found in the urine. 



Nearly all of the proteid that is destroyed is found in the form 

 of urea, uric acid, creatiniii and hippuric acid in the urine. There 

 is also found in the stools proteid which has not been digested nor 

 absorbed along the digestive tract. A part of the nitrogen is elimi- 

 nated by the desquamation of hairs, nails and epidermis. But it 

 usually suffices to determine the amount of nitrogen in the stools and 

 urine. 



If, in making up the balance, it be found that the ingesta have 

 more than equaled the egesta, it is concluded that there has been an 





