106 ANATOMY OF THE RABBIT 



for the oxidative phases of metabolism; also in the discharge of 

 waste gases, principally carbon dioxide, from the blood to the air. 

 The absorption and transport of oxygen is a specific function of 

 the red blood cells. Though the oxygen, of which a certain amount 

 always remains in the lungs during the process of breathing, must 

 pass through the thin epithelial lining of the terminal air sacs into 



Fig. 58. Photomicrograph of part of a section of the lung of a rabbit 10 micra thick. 

 X 50. (Macklin and Hartroft.) A .small venule is seen entering a larger one from the 

 right. The venule is surrounded by alveolar sacs (as), each with small, cup-like 

 aheoli (a). 



the capillaries before it can be taken into the blood cells, the latter 

 from their flattened shape and very great numbers present a rela- 

 tively enormous surface for absorption, the process being thereby 

 facilitated. Moreover, the epithelial lining referred to is so ex- 

 tremely attenuated that its completeness or even its presence in 

 the adult mammal is a matter of active dispute. The lungs them- 

 selves are highly elastic, expansible sacs. They have the structure 

 of greatly ramified saccular glands, except that the free internal 

 surfaces are everywhere in contact with air (Fig. 58). The division 



