612 BIRDS. 



The spleen lies on the right side of the proventriculus, the 

 paired thyroid lies beside the origin of the carotids, and a 

 paired thymus is found in young birds in the neck region. 

 Small yellowish (suprarenals) glands lie on the front part of 

 kidneys. 



Respiratory system. — The important facts are, — that 

 there is no true diaphragm ; that some of the bronchial 

 branches in the lungs are continued into adjacent air-sacs ; 

 that expiration is a more active process than inspiration. 



The nostrils lie at the base of the' beak overlapped by 

 the cere. Only in the kiwi are they at the tip of the beak. 

 The glottis behind the root of the tongue leads into the 

 trachea, which has a voiceless larynx at its anterior end, and 

 a syrinx, with vocal cords, at its base. The trachea is 

 strengthened by bony rings, and is moved by two sterno- 

 tracheal muscles from the sternum. The bronchial tubes 

 branch irregularly, in a kind of tree-like fashion, in the 

 lungs. These lie attached to the dorsal wall of the thorax, 

 indented by the ribs, and covered with pleural (peritoneal) 

 membrane on their ventral surface only. 



Around the lungs, and connected with the ends of the 

 main bronchial branches, are the nine air-sacs. In order 

 from behind forwards, lie the abdominals, the posterior 

 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. Their 

 chief use is to increase the bird's respiratory efficiency. In 

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

 birds the thoracic region compresses the lungs ; 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, we may almost say, the inferior 

 vena cava. But the kidney also receives some venous 

 blood from the femoral and renal-portal veins. Thus to a 

 slight extent there is a renal-portal system, which does not 



