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 lean animals, 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 55.Q 



Protein 2f.6 weight of ash ranges from 3 to 3.3 



bird's body. 



fat I7.O Protein. The term protein is 



used to designate a large group 



Ash 3Q ^ su bstances differing from other 



components of the body in that 



they contain more sulphur, com- 



Fio. 113. Graphic representa- bined USUally with about 17 per 

 tion of the approximate composi- cent Q f nitrogen. Common CXam- 

 tion of the bird s body. & . 



pies 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 



