BEEATHING 319 



a thin, elastic outer coat of connective tissue and a lining 

 of flat, mucous-membrane cells without cilia. Between 

 these two layers is a network of tiny, thin-walled capil- 

 laries whose blood is thus separated from the air in the 

 sac by only the thin capillary wall and the layer of flat 

 cells. 



The whole structure is very elastic, and readily expands 

 and contracts within the limits of the chest cavity as air 

 enters and leaves it. 



The pleura. — The pleura covers both the right and 

 left lung. This pleura is a serous membrane, and is a 

 complete cover for the lungs except at the point where 

 the bronchi enter. Here, instead of fusing with the walls 

 of the bronchi, it is turned back and hues the interior 

 of the chest cavit3^ Between the pleural cover of the 

 lung and the pleural lining of the chest cavity is thus 

 formed a cavity which is so small as to require onlj'- a few 

 drops of liquid to fill it. This cavity is filled during life 

 with a liquid of the nature of lymph, secreted by the 

 pleura, and as the lungs are inflated in breathing, this 

 liquid serves to lubricate the pleural surfaces where they 

 rub against one another and to prevent friction. 



Breathing. — Two processes are involved in breathing, 

 inhaling and exhaling. Inhaling consists of drawing the 

 air from the outside thi'ough the nasal passages, trachea, 

 bronchi, and finally distributing it through the bronchial 

 tubes until it reaches every part of the lungs and fills the 

 air sacs. Exhaling reverses this process and expels the 

 air in the air sacs through the above passages in the re- 

 verse order to the outside. As the walls of the chest cav- 

 ity enlarge in inhaling, the lungs fill with air; while in 

 exhaling, the walls contract, the lungs are compressed, 



