170 PRINCIPLES OF POULTRY FEEDING 
all these and more may be regarded as composed primarily of these 
four substances. In the body of the mature hen these are found in 
about the following proportions: Water, 55.8 per cent; ash, 3.8 per 
cent; protein, 21.6 per cent; and fat, 17 per cent (Fig. 113). 
Water.—Usually more than half, and in some birds as much as 
three-fifths, of the weight of the living bird consists of water. This 
water content is greatest in young and leananimals, and decreases as 
they become more mature or fatten. This latter feature is shown in 
the capon or soft roasters, in which the water is often as low as 40 
per cent. 
Ash.—The ash content or mineral matter is that portion of the 
body which is left after the volatile and combustible elements are 
driven off by heat. Ash is found to a limited extent in all parts, and 
it is essential to provide a sufficient amount of this material. The 
Water 558 
Protein 216 weight of ash ranges from 3 to 3.8 
per cent of the bird’s body. 
Protein.—The term protein is 
used to designate a large group 
of substances differing from other 
components of the body in that 
they contain more sulphur, com- 
Fic. 113.—Graphic representa- bined usually with about 17 per 
tion of the approximate composi- cent of nitrogen. Common exam- 
eens esyueen ples of protein are the whites of 
eggs, lean meat which has been washed free from fat particles, the 
casein in milk, and the gluten in wheat flour. Besides the elements 
already named, protein contains carbon, hydrogen, and oxygen. 
These elements are known as organic substances, as no mineral or 
ash is left after burning in air. Protein is the most important group 
of materials found in the body, as it forms the base of all living 
tissue, largely making up the material called protoplasm, which is 
the substance through which life is manifested. In the body, 
protein is always associated with the ash and water present. 
Fat.—The fourth group of materials found in the tissues of 
all birds represents a reserve value, usually in the form of fat. 
When the feed does not furnish the required supply of nutrients, 
this reserve of surplus fat helps to counteract the deficiency. The 
fatty particles in the normal body act as cushions between the 
