CHAPTER X 

 THE FUNCTIONS OF THE NUTRIENTS 



The digestion, absorption, and distribution of food 

 are not its use — ^they are the preliminaries necessary to 

 use. Not until the nutrients have been converted to 

 available forms and have passed into the blood do they 

 in the slightest degree furnish energy or buUding-material 

 to the animal organism. We have followed to a certain 

 extent the chemical changes which the digested food 

 suffers, but no detailed statements have been made as 

 to the part taken by each class of nutrients in constructing 

 the animal body and in maintaining its complex activities. 



216. General uses of food. — ^Animals use food in two 

 general ways, viz., for constructive purposes, which 

 involve the building or repair of tissue and the forma- 

 tion of milk, and as fuel for supplying different forms 

 of energy. The tissues which are to be formed are of 

 several kinds, principally the mineral portion of the bone, 

 the nitrogenous tissue of the muscles, tendons, skin, hair, 

 horn, and various organs and membranes, and the deposits 

 of fat which are quite generally distributed throughout 

 the body substance. 



216. Uses of energy. — ^Energy in the forms in which it 

 is used by the animal organism may appear as muscular 

 activity, both internal and external, such as working, 

 walking, breathing, the beating of the heart, the move- 

 ments of the stomach and intestines, as heat, and as 

 chemical energy necessary for carrying on digestion and 



(151) 



