CHAP. r. ALBUMINOIDS. 983 



consider of what materials, chemically speaking, food is made up, and 

 what are the properties and functions of these materials. This we can 

 only do briefly, referring the reader who is desirous of more complete 

 and detailed information on the subject to treatises on agricultural 

 chemistry. 



The actual chemical ingredients necessary in food are carbon, 

 hydrogen, oxygen, nitrogen, and mineral salts, for it is of these materials 

 that the animal body is built up. Some of these however, may be 

 combined in various ways, forming either nutritious substances, or 

 substances devoid of feeding value. The substances on which the 

 value of food depends are : 



(i.) Albuminoids, 

 (ii.) Fats. 



(iii.) Digestible carbohydrates (starch, sugar, &c.). 

 (iv.) Mineral salts. 



The three first named are, as regards quantity, the most important. 

 Mineral salts are vitally necessary for animals, but in foods their 

 proportion is quantitatively small compared with the first three classes 

 of substances. 



In addition to the substances just enumerated, foods contain other 

 substances which are either devoid of actual feeding value such as 

 indigestible woody fibre or cellulose, and certain organic acids, or 

 which, like the nitrogenous substances known to the chemist as 

 " amides," have only a partial or inferior value as food constituents. 



Albuminoids, are substances rich in nitrogen, as well as in carbon, 

 hydrogen, and oxygen, and are well typified by the albumin which 

 constitutes the white of eggs and by the gluten of wheat. They are 

 analogous to the substances which constitute the fibrous substance of 

 flesh or muscle, and the main solid portions of the blood, and are 

 closely related to the materials of which the cartilage of bones and 

 sinews and the substances of the skin, horns, hoofs, and hair are made 

 up. Without a due supply of albuminoids no essential animal tissue 

 can be formed, nor can the ordinary processes of life be carried on. 



The albuminoids in food are digested mainly by the pepsin in the 

 gastric juice of animals (see page 404). This converts them into soluble 

 "peptones," which are absorbed into the blood during their passage 

 through the intestines. Their function is to supply the nitrogen which 

 is essential to the building up of the various portions of the animal 

 frame to which reference has been just made, and they are thus essential 

 in liberal quantity for young growing animals, and for cows which have 

 to produce calves and to yield milk, which is a particularly nitrogenous 

 fluid. Albuminoids are also capable of producing heat and force, and 

 in some degree probably may sometimes take a direct part in the 

 formation of fat. 



The amides referred to above, of which asparagine, glutamine, 

 leucine, and tyrosine are examples, are not competent to take the 

 place of albuminoids as flesh-formers, but the oxidation of their carbon 

 and hydrogen results in the production of heat and force. 



