302 



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. 251. 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 ribs, 

 and present on their inferior surface several orifices (Fig. 251) 

 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 membranous cavities com- 

 municate 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 is 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 



