vca 



a 2 6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



shape is very like a kidney bean, and they lie with their 

 concave sides toward each other. The blood supply 



is large in propor- 

 tion to the size of 

 the organ, because, 

 as in the case of 

 the lungs, the sup- 

 ply is not mainly 

 for nutrition of the 

 organ, but for puri- 

 fication of the blood 

 (Fig. 296). The 

 blood brought back 

 from the kidneys 

 by the renal vein 

 is the purest blood 

 in the body, for it 

 is brought to the 

 kidneys as bright 

 blood purified of 

 C0 2 in the lungs, 

 but still contain- 

 ing urea ; but now 

 it is purified of its 

 urea in the kidneys 

 and returned to the 

 vena cava ascendens. There, however, it is, of course, 

 again mixed with impure blood from other tissues. 



Excretory Duct. — The excretory ducts (the ure- 

 ters) are peculiar. By a trumpet-shaped mouth each 

 ureter grasps the deeply concave part (the pelvis) of 

 the kidney so as to receive the excretions; then passes 

 down on each side as a tube about the size of a crow 

 quill and about fifteen inches long and enters the blad- 

 der on each side below by a valvular opening, which 



Fig. 296. — Showing form and position of the 

 kidneys : ao, aorta ; vca, vena cava ascen- 

 dens ; ra, renal artery ; rv, renal vein ; u, 

 ureters ; hi, bladder. 



