NUTRIENTS 171 



various organs, and also form a protecting layer under the skin, 

 giving it a plump, full appearance. The proportion of fat in the 

 bird's body varies from 15 to 34 per cent, being lowest in the adult 

 bird which is emaciated from disease or improper feeding, and 

 highest in birds which are well fattened. 



Glycogen. Another material called glycogen, very similar to 

 starch, is stored in relatively small amounts in the organs of a 

 healthy animal. This substance resembles fat in that it contains 

 neither nitrogen nor sulphur, but is composed entirely of hydrogen, 

 oxygen, and carbon, in the same proportion as in starch. It may 

 be called animal starch. 



Nutrition is the process by which life is maintained and indi- 

 vidual growth is promoted. It controls the wearing away and 

 the building up of the body tissues, converts feed into heat and 

 energy, and supplies the material for products which are essential 

 to normal life. The chief processes of nutrition are digestion, ab- 

 sorption, circulation, assimilation, and respiration. 



Nutrients. Materials in feeds, as seen in their raw state, are 

 not transformed directly into living tissue, but they are first broken 

 up into simple compounds, called nutrients, which go toward the 

 formation of the solids and fluids of the body. Their classification 

 is the same as that of the components of the body already consid- 

 ered, with an added group (carbohydrates), making five in all, 

 namely, (1) protein, (2) fat, (3) carbohydrates, (4) mineral mat- 

 ter (or ash), and (5) water. 



Protein. The group of nutrients classed as protein includes com- 

 pounds which contain those elements that are found in the tissues of 

 all plants and animals used in feeds. The classes of materials which 

 provide protein are always necessary, are the most expensive to buy, 

 and are the hardest to produce at home; hence, in purchasing feed 

 stuffs for the various rations, the proportion of protein they con- 

 tain is usually the factor which determines the price to be paid. 



Fat. In the form of oils, in seeds, in vegetable products, and 

 in animal tissues, fat is familiar to all. Most feed stuffs, especially 

 if from vegetable sources, are relatively poor in oil content. Some 

 vegetables, as flax and cotton, store up oil instead of starch and 

 are at the same time rich in protein; but, as a rule, the materials 

 commonly available for poultry feeding though poor in fat are 

 rich in carbohydrates. 



Carbohydrates. This third class of nutrients includes che 

 starches, sugars, and fibres which are chiefly valuable for producing 



