116 THE CHEMISTRY OF THE FARM 



food takes place more or less in all parts of the 

 alimentary canal, but chiefly in the small intestines. 

 The absorbed matters pass finally into the blood. 



The blood of an animal is the source of nourish- 

 ment to the whole body; out of its ingredients 

 all the tissues are formed. The blood is also the 

 means of conveying to the tissues the oxygen which 

 is essential to their vitality, and of removing from 

 them carbonic acid and the other products of their 

 decomposition. 



(3) Bespiration. — The blood is supplied with oxygen 

 during its passage through the lungs, where it is 

 brought into contact with air. The oxygen is absorbed 

 by the haemoglobin, which forms the chief constituent 

 of the red blood corpuscles. The scarlet blood from 

 the lungs is circulated through the whole body by the 

 arteries ; the oxygen it supplies is consumed in the 

 tissues, producing, among other results, heat and 

 mechanical work. The blood finally returns from the 

 tissues by the veins. The haemoglobin has then lost 

 its oxygen, and has assumed a purple colour ; the 

 plasma of the blood also contains carbonic acid gas in 

 solution, and many other products of decomposition. 

 By passing again through the lungs the carbonic acid 

 is more or less completely discharged, and a fresh 

 supply of oxygen taken up. 



(4) Excretion. — The products which result from the 

 oxidation of animal tissues, or of the food consumed, are 

 removed from the body by the lungs, the kidneys, or 

 the skin. The chief products of oxidation in the body 

 are carbonic acid, water, urea, and salts. Carbonic acid 

 is removed through the lungs, and to a smaller extent 

 by the skin ; urea and salts by the kidneys and by 

 perspiration ; water by all the organs of excretion. 



