THE COMPOSITION OF ANIMALS 23 



albumen is soluble in water, and, like other albumens, is coagulated 

 on heating above 80 C. (176 F.). Milk contains about 3.2 per 

 cent casein and albumen, the content ranging between 2.5 per cent 

 and 4.6 per cent, according 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 natural pure butter and artificial 

 or adulterated 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 makes 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 fatten- 

 ing animals, especially pigs. Adipose tissue of pigs consists of about 

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

 1.35 per cent nitrogenous substances (membrane). 



Lipoids form a group of bodies that stand close to the fats. 

 They are mixed with fats in various tissues and organs of the body. 

 Only two of these substances will be mentioned here, lecithin and 

 cholesterin. The former consists of glycerin and stearic and 

 phosphoric acids, with .a nitrogenous base known as cholin. It is 

 found in the nerve tissues, cellular structures, and in the bile. 

 Cholesterin is likewise found in the nerve tissues and cells of the 

 body, and also in the liver, brain, eggs, and in wool fat. It is 

 composed of carbon, hydrogen, and oxygen, and does not contain 

 either nitrogen or phosphorus. In spite of relatively small quanti- 



