GROWTH OF ANIMALS; ANIMAL FOOD. 139 



the various kinds of animals, which is a whitish, oily 

 substance, differs but little in appearance, though some- 

 what in composition. 



Nitrogenous Substances. — These consist chiefly of 

 carbon, hydrogen, oxygen, and nitrogen, though phos- 

 phorus and sulphur are always present in small amounts. 

 This class may be subdivided into three groups : 1. Albu- 

 minoids ; 2. Gelatinoids ; 3. Horny Matter. 



Albuminoids. — These are the most important, because 

 animal life is dependent chiefly upon them and the or- 

 gans composed of them, and because they fm-nish the 

 material out of which the other groups are formed. They 

 are found in various forms in the body, the chief of 

 which are albumen, fibrin, and casein. These, while dif- 

 fering widely in appearance, agree in their chemical com- 

 position, in that each in a pure, dry state contains about 

 sixteen per cent of nitrogen, and from one to one and 

 five-tenths per cent of sulphur; albumen is represented 

 by the white of egg, fibrin by the white solid remaining 

 after the red color of coagulated blood is washed out, 

 while casein forms the basis of cheese. 



Gelatinoids. — These form the nitrogenous substances 

 of bone, skin, and cartilage, and the connective tissue of 

 the animal body. They may be extracted by boiling 

 with water ; the resultant product is glue. Their com- 

 position is similar to the albuminoids, in that on the 

 average they contain about sixteen per cent nitrogen, 

 the cartilage containing less, and the bones and skin 

 more, nitrogen than the albuminoids. 



Horny Matter. — The hair, horn, hoofs, claws, nails, 

 wool, and feathers are constituted mainly of horny matter. 



