CLASS AVES. BIRDS 



441 



Trachea- 



Cervical air sac 



Interclavicular air sac 



Bronchus- 



Anterior 

 thoracic air sac 



Lung 



Posterior 

 thoracic air sac 



Abdonninal air sac 



Syrinx 



Air sac in 

 cavity of 

 humerus 



Figure 311. Diagram of the respiratory organs of a pigeon, showing the air sacs. (After Thom- 

 son's Outlines of Zoology revised by James Ritchie. Ninth edition. Copyright 1944 by Oxford 

 University Press.) 



serve in temperature regulation by getting 

 rid of excess bodv heat, since in birds there 

 are no sweat glands. The air sacs do not re- 

 duce the specific gravity of the body as is 

 commonly claimed. 



The trachea is held open by partially cal- 

 cified cartilaginous rings. Where the trachea 

 at its lower end divides into the two bronchi, 

 it enlarges to form the vocal organ or syrinx, 

 a structure peculiar to birds. Extending for- 

 ward from the angle of bifurcation of the 

 trachea is a semilunar membrane which is 

 vibrated like a reed when air is forcibly ex- 

 pelled from the lungs, thus helping to pro- 

 duce sound. The different songs and calls 

 of birds are made possible by variations in 

 the sound mechanism. 



Because of the high rate of life processes 



in the active bird, the demands for oxygen 

 are exceedingly great, but they can adjust 

 themselves remarkably well to varying en- 

 vironmental conditions with adaptations 

 that enable them to conserve energy during 

 cold weather, or when sleeping. 



Excretory system 



The kidneys are a pair of dark brown 

 three-lobed bodies situated as shown in Fig. 

 306. Each discharges its semisolid nitrog- 

 enous waste (mostly urates) through a duct, 

 the ureter, into the cloaca. There is no 

 urinary bladder, but the semisolid "urine" 

 passes directly out of the cloaca with the 

 feces and appears as a whitish substance on 

 the latter. 



