RESPIRATION. 85 



The absorption of the dissolved constituents of the 

 food takes place more or less in all parts of the alimentary 

 canal, but chiefly in the small intestines. The absorbed 

 matters pass 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 meta- 

 morphosis. 



3. Respiration. 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 thus produced 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 blood serum also contains carbonic 

 acid gas in solution, and many other products of decom- 

 position. By passing again through the lungs the car- 

 bonic acid is more or less completely discharged, and a 

 fresh supply of oxygen obtained. 



4. Excretion. The products which result from the 

 oxidation of tissue, 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 ; 



