28 DISKASKS Ol- I-OILTKY. 



found in no other class of animals and is situated 

 where the trachea divides to form ])ronchi. 



The two primary bronchi, formed by the division of 

 the trachea, penetrate the lungs, branching as they go 

 and end in the air sacs which have already been men- 

 tioned. The primary branches of the bronchial tubes 

 give off secondarj^ branches, and the secondary branches 

 iu turn send off tertiary branches. These finer branches 

 open upon a dense network of minute, almost naked, 

 blood vessels, through the delicate walls of which the 

 oxj-gen passes to the blood and the carbonic acid and 

 other vapors escape. 



There are in all nine air sacs, four on each side, and 

 one (the interclavicular) which has been formed b}' the 

 union of another pair. These are named the cervical, 

 the interclavicular, the anterior thoracic, the posterior 

 thoracic, and the abdominal sacs. With many l)irds 

 the air sacs extend into the humerus or large bone of 

 the wing, w^th some, into the breastbone, the thigh 

 l)one, and even other bones of the body. The air sacs 

 act principallj' as reservoirs of air, feeding the lungs 

 between the periods of inspiration and ensuring an 

 abundant air supply at all times. There is possiblj' 

 some oxygenation of the blood through their walls, 

 and the)^ serve in addition to reduce the weight of the 

 body. 



Nearly all the moisture w'hich escapes from the bird's 

 body passes off through the respiratory organs. With 

 mammals, a large proportion of the liquid taken into 

 the body is evaporated from the surface of the skin, 

 and this process serves as an efficient regulation of the 

 body temi)erature. A considerable amount of liquid is 

 also excreted by the kidneys of such animals. In ])irds 

 the secretion of the kidnej's contains but a small quan- 



