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 fire 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- 
