2 BULLETIN 459, U. S. DEPARTMENT OF AGRICULTURE. 



proportion of water is greatest in young and lean animals and 

 decreases as they become more mature or fatter. 



Ash. — The ash or mineral matter is the portion left after complete 

 burning. Its presence is most familiar in the bones, but it is found 

 in all parts of the body and is just as essential as water or protein. 

 It amounts to from 2 to 5 per cent of the weight of the body. 



Protein. — Protein is the name given to a highly important group 

 of substances, of which the white of egg, washed lean meat, the 

 casein of milk, the gluten of wheat flour, etc., are familiar examples. 

 They are composed of the chemical elements carbon, hydrogen, 

 oxygen, nitrogen, and sulphur. They are what are commonly called 

 "organic" substances, which simply means that they maybe burned 

 completely in air or oxygen. They differ from the other groups 

 of substances found in the animal body in containing sulphur, and 

 especially nitrogen, the latter element constituting from 15 to 18 or 

 19 per cent of their weight. 



Protein is the basis of the living tissues of the body — the so-called 

 protoplasm — and is the substance through which life especially 

 manifests itself. In the body it is always associated with water 

 and ash. 



THE RESERVE MATERIAL OP THE BODY 



Fat. — Besides its working parts, the body contains a store of 

 reserve material in the form of fat. While the fat deposits in the 

 body are of use mechanically as cushions between the various organs 

 and as a protecting layer under the skin, nevertheless fat represents 

 essentially a storage of material derived from feed consumed in 

 excess of the body's immediate needs. When the feed is insufficient 

 or entirely lacking, this store of surplus material is drawn upon, 

 and the animal gradually becomes lean. The percentage of fat in 

 the bodies of agricultural animals may vary greatly but seldom 

 falls below 6 or rises above 30 per cent. 



Glycogen. — Besides fat there are stored up in the muscles, liver, and 

 other organs of a healthy animal rather small amounts of a substance 

 called "glycogen," belonging to the group of carbohydrates described 

 in the next section. Neither fat nor glycogen contahis the elements 

 nitrogen or sulphur, but each is composed entirely of carbon, hydro- 

 gen, and oxygen. 



COMPOSITION OF THE ENTIRE BODY. 



The average results of analyses shown in the following table 

 indicate the composition of the bodies of different animals in different 

 conditions: 



