NUTRITION 265 



have either escaped digestion or are undigestible, the intake 

 of the body may be ascertained by substracting the food nu- 

 trients consumed in twenty-four hours and the nutrients pass- 

 ing off in the same length of time undigested and unabsorbed. 



When the intake of the body balances the outgo, the bird 

 neither loses or gains in weight, and an equilibrium is estab- 

 lished. If the intake exceeds the expenditures the bird gains 

 weight; if the intake is less than the expenditures the bird loses 

 weight. 



Metabolism. — By metabohsm we mean the changes taking 

 place in the living tissue. 



Every muscular contraction, the beating of the heart, the 

 muscular movements of the bowels and oviduct, movements in 

 respiration, all mean wear and tear or tissue waste and must be 

 repaired as rapidly as the part is destroyed. The process of 

 tearing down or destruction is called kataholism, and that of 

 building up, anaholism. 



It is quite apparent that metabohsm is under the influence 

 of the nervous system. 



The elements required to make good this tissue" waste must 

 be contained in the food. These elements are in the form of 

 water, ash, protein (nitrogen-containing nutrient), fats and 

 oils (hydrocarbons), and starches and sugars (carbohydrates). 

 Each of these groups of food nutrients must be in the proper 

 proportion for the purpose for which the birds are being fed. 



The nitrogenous nutrients digested, absorbed, and carried 

 to the cells of the body, as told under the subject of digestion, is 

 not stored up in the body, but immediately used. Voit con- 

 tends that the digested protein after being absorbed is divided 

 or split up in the tissues into two portions. One portion re- 

 pairing the cells injured in wear and tear and the other circu- 

 lating in the blood and lymph, bathing the cells, is destroyed 

 in combustion in the production of heat and energy and the 

 nitrogen end-products which are eliminated in the form of uric 

 acid by the kidneys. 



In the young and developing animal the protein is also used 

 in nitrogenous tissue growth and expansion. 



To summarize, we may say that the body, whether it be 

 mature, growing, in egg production, or fattening, needs nitro- 



