CHAPTER XI 



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 building 

 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 nutri- 

 ents in constructing the animal body and in maintain- 

 ing its complex activities. 



Animals use food in two general ways; viz., for 

 constructive purposes, which involve the building or 

 repair of tissue and the formation of milk, and as 

 fuel for supplying different forms of energy, including 

 heat. The tissues which are to be formed are of sev- 

 eral 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. 



Energy in the forms in which it is used by the ani- 

 mal organism may appear as muscular activity, such as 



(151) 



