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 may be 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 OF 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 contains 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: 
