134 THE BIOLOGY OF BIRDS 



interesting in their close correspondence, in nature and in 

 proportions, with the sahs of the sea. This is surely a hint 

 that blood-containing animals began their evolution in the 

 sea. Birds sprang from reptiles, these from amphibians, 

 and these from fishes, which bring us to the sea. The 

 difference in proportion between the salts in the sea and the 

 salts in the blood of a higher animal is lessened when wc 

 take into account the change that has occurred in the com- 

 position of sea-water since the time of the primeval ocean 

 when blood was first established. 



The chief uses of the blood are five : — 



(i) The blood is a gas carrier. It carries oxygen from 

 the place of capture (the bird's lungs) to the place of com- 

 bustion (the muscles and other tissues). The oxygen is 

 mainly carried by the red blood corpuscles, whose pigment 

 haemoglobin enters readily into a loose union with oxygen. 

 But the blood also carries carbon dioxide from the place 

 of production (the muscles and any other active tissue) to 

 the place of elimination (the lungs). The carbon dioxide 

 is mainly carried by the serum of the blood, e.g. in union 

 with sodium salts. 



(2) The blood carries and distributes the digested 

 proteins (changed into peptones, etc.) and the digested carbo- 

 hydrates (changed into sugar). These products of digestion 

 are absorbed from the walls of the stomach and intestine, 

 and after passing through the liver are distributed through 

 the whole body. The digested fat (changed into fatty 

 acids and glycerol) is first collected by the lymphatic 

 system, which eventually communicates with the veins 

 and the general circulation. 



(3) With the help of the lymph the blood collects the 

 soluble nitrogenous waste-products of the body, and these 

 are eliminated by processes beginning in the liver and 

 completed in the kidneys. 



(4) The blood also distributes through the body the 

 potent internal secretions of the ductless glands, such as 

 the thyroid and the supra-renal. These will be discussed 

 in a subsequent paragraph. 



