148 The Feeding of Animals 



complete. Here the proteids may be partially burned 

 to carbonic acid and water, but a portion of their sub- 

 stance passes from the body principally in the form of 

 urea and uric acid, which are the prominent constituents 

 of urine. These compounds carry with them a certain 

 proportion of carbon and hydrogen which in ordinary 

 fuel combustion would more fully unite with oxygen. 

 The heat production from protein is therefore less in 

 the animal than in the furnace. 



This oxidation in the animal is constant but not 

 uniform. It varies with the exercise the animal is tak- 

 ing and with the amount of food that must be disposed 

 of. The quantity of oxygen needed is therefore vari- 

 able, and when the demand for it is largely increased 

 the heart pumps faster, more blood passes through 

 the lungs, the breathing is more rapid and the supply 

 of oxygen is in this way augmented. 



ELIMINATION OF WASTES 



The various waste products from this combustion 

 and from the breaking up of the proteids within the 

 animal evidently must be disposed of in some manner. 

 If not eliminated from the body, they would cause re- 

 sults of a most serious character, as, for instance, 

 when an accumulation of urea in the body produces 

 uraemic poisoning. The blood therefore not only carries 

 to the tissues the necessary nutrients and oxygen, but 

 it has laid upon it the burden of taking into its cur- 

 rents the waste products of combustion and growth and 

 carrying them to the points where they are thrown off. 



