EEPAIE OF WASTE. 47 



kidneys and skin eliminate water ; the degraded nitro- 

 genous matters, and broken-np mineral substances 

 being 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 

 the 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 propor- 

 tions, 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 following tables may serve as a guide to the 

 reader : 



