132 THE BIOLOGY OF BIRDS 



The two kidneys of birds are three-lobed red organs, 

 which He embedded underneath the hip-girdle. They 

 receive their blood mainly from renal arteries and they 

 return it to renal veins. These join the great posterior 

 vein, the inferior vena cava, leading back to the heart. 



§ 7. The Uses of the Blood 



The blood permeates the whole body, carried from the 

 heart by arteries, carried back to the heart by veins. Fine 

 branches of the arteries — arterial capillaries — become con- 

 nected with fine tributaries of the veins — venous capillaries, 

 so that the blood circulates in a continuous system. There 

 is diffusion from the blood into the tissues traversed, and 

 the tissues are even more directly bathed by the lymph 

 fluid which moves in channels and vessels, the latter 

 eventually communicating with the venous system. The 

 blood is a common medium for the whole body, from which 

 all parts take, and to which all parts give. It is more to 

 the body than the Nile to Egypt. 



Let us inquire, then, what the bird's blood is and what 

 its uses are. 



The blood consists of a complex fluid or serum in 

 which there float red blood-cells and colourless blood-cells. 

 The former, the erythrocytes, are nucleated cells or 

 corpuscles, ellipitical in outline, and slightly biconvex ; 

 thus difi^ering markedly from our own, which are non- 

 nucleated, circular in outline, and slightly biconcave. The 

 members of the camel family are the only mammals where 

 the red blood corpuscles have the elliptical shape seen in 

 birds and reptiles, amphibians and fishes. As to the white 

 blood corpuscles or leucocytes, they are nucleated amoeboid 

 cells, often changing their shape, and there are several 

 different varieties. Among these kinds are the phagocytes, 

 which are prone to leave the blood and migrate in the 

 tissues, engulfing microbes, devouring degenerating tissue, 

 transporting material from one area to another, and helping 

 to repair injuries. They form a body-guard against in- 

 vading bacteria ; whenever there is inflammation they are 



