42 THE STOCKFEEDER'S COMPANION 



air that has been drawn into the air cavities of the lungs. 

 Here the blood takes up the oxygen in the air, and 

 liberates the carbon dioxide gas due to the red 

 colouring matter (haemoglobin) having a much stronger 

 affinity for oxygen than carbon dioxide gas. The blood 

 then changes to a bright scarlet colour and the carbon 

 dioxide gas escapes from the lungs in the breath, along 

 with considerable quantities of water-vapour. In this 

 way the lungs are continually ridding the blood of this 

 injurious product of oxidation. At the same time the 

 blood is being charged with oxygen, which it carries 

 round the body, giving it up wherever it is required to 

 oxidise the food for the production of heat or work, etc. 



Excretion. The chief impurities of the blood 

 which are not removed by the lungs are urea, certain 

 salts, and water. Urea is formed from the decomposi- 

 tion of nitrogenous matter in the blood. The carbon 

 and hydrogen alone are oxidised to carbon dioxide and 

 water respectively. The nitrogen, however, forms urea, 

 and is removed by the kidneys as well as through the 

 pores of the skin (perspiration). 



When the urine is collected, and the amount of 

 nitrogen in it determined, one can tell by difference 

 from the total food digested, the amount of albuminoids 

 which have been stored in the body. The difference 

 between the compositions of food and solid excrement 

 gives the amount digested by the animal. 



The various salts contained in the food which are 

 not required by the body are got rid of in the 

 urine, and to some extent in the perspiration. Soda 

 and potash are common bases in these salts, because 

 they are fairly abundant in the food given to farm 

 animals, and produce salts which are easily dissolved. 



The undigested food is expelled from the body in 



