754 PHYLUM CHORDATA : CLASS AVES — BIRDS 



thoracics, the anterior thoracics, the cervicals, and the inter- 

 clavicular in the middle line in front. The interclavicular 

 sac is in connection with both lungs, and is continued into 

 two axillary sacs in the arm-pits. The anterior and posterior 

 air-sacs are continuous with air-spaces in the bones. In 

 the resting bird the sternum rises and falls ; in the flying 

 bird the thoracic region compresses the lungs and air-sacs ; 

 in either case, expiration is the more active part of the 

 respiratory process. 



Excretory system. — The kidneys are three-lobed, and 

 lie embedded in the pelvis. They receive blood from the 

 dorsal aorta by renal arteries, and the filtered blood leaves 

 them by renal veins which unite with femorals and renal 

 portals to form the iliacs, or, w^e may almost say, the inferior 

 vena cava. But the kidney also receives a little venous 

 blood from branches of the femoral veins. Thus, there is 

 just a hint of a renal-portal system, which does not occur 

 in Mammals. The kidneys are metanephric in origin. 



The waste products, consisting for the most part of urates, pass in 

 semi-solid form down the ureters into the median compartment of the 

 cloaca. 



In front of each kidney, at the base of the iliac vein, there lies a 

 suprarenal body. 



Reproductive system. — The testes lie in front of the 

 kidneys. Like the ovary, they increase in size at the 

 breeding season, and dwindle afterwards ; the sexual 

 period in birds being much more narrowly limited than in 

 most other Vertebrates. 



The spermatozoa pass from the testis into a vas deferens, 

 which lies to the outside of the corresponding ureter. The 

 vasa deferentia, slightly convoluted when full of sperms, and 

 with a posterior swelling or seminal vesicle, open separately 

 into the median compartment of the cloaca. 



In the adult pigeon, and in most birds, there is only one 

 ovary ; that of the right side usually atrophies early in life. 

 The right oviduct is represented by a small rudiment close 

 to the cloaca. 



The ovary is covered with follicles containing ova at 

 various stages of ripeness. As these ova become dilated 

 with yolk and otherwise mature, they burst from the ovary, 

 and are caught by the expanded end of the oviduct which 



