REPAIR OF WASTE. 37 



The carbonic acid, as we have already seen, is got rid 

 of by means of the lungs, which, together with the 

 kidneys and skin, eliminate water ; while the degraded 

 nitrogenous matters, and broken up mineral substances 

 are excreted by the kidneys. 



Repair of Waste. In order to supply material for the 

 repair of the constant waste experienced by the system, 

 the animal must have food which shall answer the 

 following conditions : 



1. It must contain a proper proportion of the 



elements necessary for the building up of new 

 tissue. 



2. It must be digestible. 



3. It must be of sufficient bulk for the stomach and 



intestines to act on it. 



4. It must be palatable. 



Grain, grasses, and roots, used in suitable proportions, 

 answer the above conditions. As chemistry enables us 

 to analyse these foods, we may, by its aid, approximately 

 judge of their respective nutritive values. The follow- 

 ing tables may serve as a guide to the reader : 



