RESPIRATORY AND EXCRETORY SYSTEMS. 611 



or great omentum which covers the abdominal viscera. It leads not 

 into the liver, but into one of the hepatic veins. 



Associated with the blood vascular system, there is a 

 lymphatic system with a few lymphatic glands. The spleen 

 lies on the right side of the proventriculus, the paired 

 thyroid lies beside the origin of the carotid arteries, and a 

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



Respiratory System. 



The important facts are, that there is as yet no diaphragm, 

 that some of the bronchial branches in the lungs are con- 

 tinued 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 trachea is strengthened by bony rings, and is moved 

 by two sterno-tracheal muscles from the sternum. It has a 

 larynx at its anterior end, and a syrinx, with vocal cords, at 

 its lower end, where the bronchi diverge. The bronchial 

 tubes branch in a kind of tree-like fashion in the lungs. 

 These lie attached to the dorsal wall of the thorax, indented 

 by six of the ribs, and covered with pleural membrane on 

 their ventral surface only. 



Around the lungs, and connected with the ends of some 

 of the bronchial tubes, are nine air sacs, four lateral and 

 one median. In order from behind forwards, lie posterior 

 or abdominal sacs, the posterior thoracics, the anterior 

 thoracics, the cervicals, and the interclavicular in the middle 

 line in front. The anterior and posterior air sacs are con- 

 tinuous with air spaces in the bones. 



Excretory System. 



The kidneys are three lobed, and lie embedded in the 

 ilia. 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 venous blood from the sciatic and 

 other posterior veins. Thus to some extent there is a renal 

 portal system, which does not occur in Mammals. 



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

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



