THE COMPOSITION OF ANIMALS 23 



cent casein and albumen, ranging between 2.5 and 4.6 per cent ac- 

 cording to the quality of the milk; about 80 per cent of the total 

 milk proteins is composed of casein ; the rest is largely albumen. 



Fats may be present in animals as body fat, in the marrow of 

 bones, and in milk. They occur in the former two as oval or round 

 cells that are composed of a nitrogenous membrane filled with fluid 

 fat in live animals. The body fat is similar in composition to the 

 vegetable fats, being largely composed of glycerides of the fatty 

 acids, stearic, palmitic, and oleic acids, but the proportions of the 

 different glycerides vary from that of plant fats, and there are also 

 characteristic components of animal fat which are not found in the 

 vegetable kingdom. Milk fat is composed of the three glycerides 

 mentioned 'and, in addition, of about 8 per cent of glycerides of 

 volatile fatty acids (mainly butyric acid), which give the char- 

 acteristic fine flavor to fresh butter and, on decomposition, a ran- 

 cid flavor to old butter. On account of the presence of these volatile 

 fatty acids in butter it is possible to distinguish, by means of 

 chemical analysis, between pure and artificial butter. 



Body fat may be deposited in animals receiving an abundant 

 . supply of feed ; it is stored either between the layers of muscular 

 tissue, about the internal organs, or directly beneath the skin, espe- 

 cially on the backs of animals. The body fat forms a reserve ma- 

 terial that the animal can draw upon in time of a scarcity of feed. 

 Through systematic liberal feeding and other favorable conditions 

 the faculty to lay on body fat has been greatly developed in fattening 

 animals, especially pigs. Adipose tissue of pigs consists of about &2 

 per cent of pure fat, the balance being 6.4 per cent water and 1.35 

 per cent nitrogenous substances (membrane). 



Lipoids are organic substances closely related to the fats and 

 found mixed with these in various tissues and organs of the body. 

 The two most important lipoids are; lecithin found in egg yolks, 

 blood, lymph, and cholesterol, present in bile, liver, eggs and wool 

 fat; both are also normal constituents of brain and nerve tissue. 



Vitamins are important organic substances of unknown com- 

 position that have been recently identified in minute quantities in 

 many feed materials. One vitamin, which is soluble in fat, is found 

 associated with certain animal and vegetable fats, like butter fat, 

 fat of egg yolk, kidney and liver fat, as well as in the leaves of 

 plants and, in small amounts, in cereal grains. Another vitamin is 

 soluble in water and is found in the leafy portion of plants and 

 vegetables, in seeds, milk, eggs, etc. A third vitamin is present in 

 vegetables and fresh fruits, especially in orange juice and to- 

 matoes; it prevents the development of scurvy and is therefore 

 called antiscorbutic. 



