PBOCESSES OF EXCEETION 117 



Non-nitrogenous substances, as fat and sugar, when 

 oxidised in the body, yield simply water and carbonic 

 acid. The nitrogen of the albuminoids, gelatinoids, 

 and amides is not oxidised, but is excreted in the form 

 of urea. The sulphur of the albuminoids is in part 

 oxidised to sulphuric acid and excreted as sulphates. 



The quantity of nitrogen in the urine is a measure 

 of the albuminoids, gelatinoids, and amides oxidised in 

 the body. In the urine, and in the perspiration of the 

 skin, are also removed all the salts not required for the 

 animal economy. Sodium and potassium salts are 

 generally abundant in the urine. 



The solid excrement contains the undigested part of 

 the food, with the residues of the bile and other secre- 

 tions of the alimentary canal. When an animal is 

 supplied with known quantities of food per day, the 

 composition of which has been ascertained by chemical 

 analysis, it is possible by collecting the faeces, and sub- 

 mitting them to the same chemical analysis, to deter- 

 mine how much of each constituent of the food has 

 been digested by the animal (see p. 141). 



