298 POULTRY PRODUCTION 



tliat unly in the severest weatlier is feed used direetly as a 

 Sduree of lieat. 



A considerable amount of muscular work must be done if 

 the bird is to Hve. It must at least make the motions neces- 

 sary for eating, and most individuals find it necessary to 

 move about in the search of feed. This is done l>y the fowl's 

 own volition, and such movements are termed voluntary. 

 At the same time there is considerable muscular activity 

 within the body of the fowl over which it has no control, 

 and which is termed in\'oluntary. Such motion is found in 

 the contraction ol the gizzard and intestines in ])reparing 

 and digesting the feed. Energy must be furnished for both 

 voluntary and involuntary motion. 



All the tissues of the body, whether active or at rest, are 

 continually breaking down and must be replaced. Material 

 for replacement must be furnisheil. 



In order that the joints and nuisidcs may move with little 

 friction and that the egg may slip down the oviduct to a 

 successful exclusion there must be lubricating material. 

 And in order that feed may be digested there must be diges- 

 tive juices. All these call for the elaboration of secretions of 

 various sorts for whicli there must be material. 



It is a recent disco\'ery that for the proper performance 

 of the various organ systems of the body, even f(.)r mainten- 

 ance, certain substances called vitamins, the chemical 

 composition and precise function of which are not understood, 

 are necessary. 



The source of the sui)i)ly of energy, the material for new 

 tissues and secretions and of the vitamins, can only be the 

 feed. A certain ])art of the feed, therefore, must alwa\s l)e 

 used for fuel, repairs, and lubricants; that is to say, for 

 the maintenance of life. With hens in full laying it takes 

 amounts varying from a little less than three-quarters to 

 about four-hfths of the feed supplied to meet the needs of 

 maintenance alone.' 



Growth. — In reaching a normal size a chick is said to grow. 

 Growth, which is primarily an increase in bone and protein 



' See Tables XXXIV :iii.| XLI 



