332 



ZOOLOGY. 



action of the oxygen contained in these cells, as well as whilst 

 traversing the pulmonary capillaries. 



Trachea. 



Pulmonary 

 Vessels. 



Bronchus 

 "laid open. 



Bronchus 

 " laid open. 



Fig. 317. Lungs of a Bird. 



The lungs are not divided into lobes, neither do they fill 

 the cavity of the thorax. They are, as it were, fixed to the 

 rihs, and present on their inferior surface several orifices 

 (Fig. 317) belonging to the bronchial tubes, which traverse 

 them through and through, and thus convey the air into the 

 various air cells spread throughout the body. These mem- 

 branous cavities communicate with each other. 



The extension of these cells, and consequently of the air 

 they contain, bears a ratio to the powers of flight of the bird : 

 in the eagle they are found in all the bones ; in the penguins 

 the air is excluded from all, or from nearly'all, the bones. The 

 air js generally found to extend most into the bones chiefly 

 used for locomotion, as the femur of the ostrich. 



We have already alluded to the power which birds have of 

 resisting cold, due to the development of the respiratory 

 function, and to a higher temperature than is found in other 

 animals. 



441. As in mammals, the organ of voice is a dependence 

 of the respiratory system. The upper larynx is of a very 



