FOOD. 133 



Of these elementary bodies, carbon and nitrogen are considered 

 especially important ; carbon as forming the most abundant and char- 

 acteristic ingredient of all organic combinations, and nitrogen as the 

 distinguishing element of albuminous substances. Of these two, the 

 .system requires daily, in an active condition, about 20 grammes of 

 nitrogen and about 280 grammes of carbon. This alone makes it 

 evident that a mixed diet of animal and vegetable food is the most 

 available for man. Meat contains, according to Payen, 3 per cent, of 

 nitrogen and 11 per cent, of carbon. Consequently, if the diet were 

 composed exclusively of this food, the necessary quantity of nitrogen 

 would be supplied by 666 grammes of meat ; but in order to obtain the 

 required carbon, 2,545 grammes would need to be consumed, thus in- 

 volving a waste of its nitrogenous matter. On the other hand, bread, 

 the most nutritious of vegetable substances, contains 1 per cent, of 

 nitrogen and 30 per cent, of carbon. Therefore, if this were the only 

 food used, 933 grammes would be sufficient to supply all the carbon ; 

 but, in order to obtain the due amount of nitrogen, it would be 

 necessary to consume 2,000 grammes. A mixture, accordingly, of the 

 two kinds of food, in which nitrogenous and hydrocarbonaceous matters 

 respectively preponderate, is best adapted to supply the wants of the 

 system without unnecessary expenditure of material. 



The changes undergone in the body by the ingredients of the food, 

 and their final destination, vary for different kinds. The carbohydrates 

 no doubt, after serving their purpose in the animal economy, are 

 finally expelled under the form of carbonic acid and water. The 

 oxygen, introduced with the inspired air, produces this result by unit- 

 ing with the carbon of the organic body, while its hydrogen and oxygen, 

 already present in the relative quantities to produce water, are liberated 

 under that form. This result is expressed by the following formula : 



Starch. Carbonic acid. Water. 



C 6 H 10 5 + O w = 6(C0 2 ) + 5(H 2 0). 



Thus the change undergone by starch and allied substances in the 

 animal body, where they are consumed, is precisely the reverse of that 

 taking place in the act of vegetation, by which they are produced. 



For the fats the change is a similar one, their only final prod- 

 ucts, so far as we know, being carbonic acid and water. But for 

 this they require, as already mentioned, a greater supply of extra- 

 neous oxygen, since, beside their larger proportion of carbon, they 

 also contain hydrogen which requires further oxidation, to form water. 

 The change thus undergone by fatty substances may be expressed as 

 follows : 



Fat. Carbonic acid. Water. 



CsiH^Oe + 145 = 51(00.) + 49(H 2 0). 



In the case of albuminous matters the process is a different one. 

 These substances contain an element, namely, nitrogen, which does not 

 appear in the carbonic acid and watery vapor of the expired breath, 



