CHAP, xxil.] CLASSIFICATION OF FOOD. 153 



Thus, milk consists of water, sugar, oily matters (butter), caseine ; 

 and wheat, a substance of almost universal application for food, ex- 

 hibits an analogous union of starch, the representative of the saccha- 

 rine group and of gluten, representing the albuminous. 



It must be borne in mind that the albuminous aliments are dis- 

 tinguished from those of other groups by their containing nitrogen. 

 Food of this kind is especially fitted to be directly assimilated to 

 muscle, nerve, and the other animal tissues, into the composition of 

 which nitrogen enters largely. These aliments contribute directly 

 to the formation of the blood, from which the tissues attract the 

 principles most proper for their nourishment. 



The non-nitrogenous aliments are obviously fitted to nourish those 

 textures which do not contain nitrogen, as the fat, or to supply 

 those secretions in which carbon abounds, as the bile. Moreover, 

 they furnish those large supplies of carbon which we are warranted 

 in supposing the animal economy stands greatly in need of, not 

 only from the great amount of that element which is to be found in 

 all the tissues, but also from the large excretion of carbonic acid 

 which is constantly taking place from the respiratory and other 

 surfaces of the animal body. The formation of carbonic acid in the 

 O3conomy by the union of carbon and oxygen is, no doubt, the im- 

 mediate cause of the generation of animal heat, and thus the supply 

 of carbon in the food becomes of great importance to the mainte- 

 nance of the proper temperature of animals. 



From the natural subdivision of the food of man into two classes 

 one, consisting of the nitrogenized matters, well adapted by their 

 constitution for the formation of blood ; and the other, the non- 

 nitrogenized substances, serving to supply a large amount of carbon, 

 Liebig proposes to name the former the plastic elements of nutrition, 

 arid the latter elements of respiration. 



To the first term we see no objection buttheuseof any term which 

 would imply that respiration must be, as it were, fed directly through 

 the digestive process, appears to us scarcely consistent with the real 

 facts of the case. The respiratory process is partly a process of sup- 

 ply, and partly one of depuration. It supplies oxygen, and it assists 

 in the removal of effete matters in the shape of carbonic acid. The 

 effete particles of the tissues would probably supply sufficient car- 

 bonic acid to effect the attraction of the required amount of oxygen 

 but as the supply of oxygen has the ulterior object of generating a 

 due amount of heat, there will be required for this purpose a larger 

 quantity of carbon than can be obtained merely from the destructive 

 assimilation of the tissues (to use Dr. Front's expression). Hence, 



