130 FOOD. 



The remaining substance nuiy then undergo complete oxidation 

 without the further production of any nitrogenous compound. This 

 double result of the decomposition of the albuminous substances, to- 

 gether with the fact that we take habitually between four and five 

 times as much non-nitrogenous as nitrogenous matter in the food, will 

 explain the great preponderance in quantity of carbonic acid as an 

 excretion over urea. For while the average daily quantity of urea 

 discharged is only 35 grammes, the carbonic acid exhaled with the 

 breath amounts to from 700 to 800 grammes per day ; the entire quan- 

 tity of the carbonic acid produced being, by weight, fully twenty times 

 as great as that of the urea. Urea is a nitrogenous substance sepa- 

 rated by decomposition from the albuminous ingredients of the system ; 

 while carbonic acid represents the combination of its remaining elements 

 with the abundant oxygen introduced by the breath. 



The quantities of the various substances taken in with the food and 

 discharged with the excretions are liable to many variations from the 

 changing condition of the individual. If the body be increasing in 

 weight, the substances introduced will be greater in quantity than those 

 discharged ; if it be diminishing, the material discharged will be more 

 than that introduced. Even in the healthy adult, where the body does 

 not sensibly gain or lose weight for long intervals, observation has 

 shown that there are frequent fluctuations of small extent, and that the 

 income for any single day rarely counterbalances exactly the outgo for 

 the same period. Consequently the quantities given in the preceding 

 tables cannot be taken as furnishing, in any case, a uniform and 

 invariable standard, but only as showing what, upon the whole, are the 

 relative proportions of the different ingredients entering into the com- 

 position of the food and of the bodily frame. And although for many 

 of them we are not yet able to ascertain their quantities with sufficient 

 accuracy for determining all the changes which they undergo in the 

 system, yet there is no doubt of the main result produced by the internal 

 transformation of the ingredients of the food. We have certain nutri- 

 tious substances introduced on the one hand, and certain excrementitious 

 products discharged on the other, which may be expressed as follows : 



INTRODUCED WITH THE FOOD. DISCHARGED WITH THE EXCRETIONS. 



Albuminous matter. Urea. 



Fat. Carbonic acid. 



Carbohydrates. Water. 



This represents the decomposition and metamorphosis of the organic 

 substances proper ; while the mineral ingredients of the food, as a rule, 

 pass through the system unchanged. 



