EESPIEATOEY SYSTEM. 



51 



The lungs are two in number, situated in the chest, and 

 each is enveloped with a membrane called the pleura. The 

 cavity of the chest is also lined with the same sort of mem- 

 brane. The upper or anterior end of the air-tube opens 

 into the pharynx. The lower or posterior part divides into 

 two branches called the bronchial tubes, one of which passes 

 to each lung and there divides and subdivides into a o- re at 

 number of branches, as seen in Fig. 58. 



The lungs are made up of a vast number of small cavi- 

 ties or cells, into each one of which a branch of the bron- 



FIG. 59. 



Small portion of Human lung. Magnified. 



chial tube opens ; and it is in these cells that the air acts 

 upon the blood contained in the capillary vessels, above 

 mentioned, and renders it pure. 



The act of breathing is performed by the involuntary 

 movements of the walls of the chest, including that of the 

 diaphragm, that is, the broad muscle which separates the 

 cavity of the chest from that of the abdomen. 



During ordinary circumstances there enters into the 

 lungs of a man about five and a half quarts of air per 

 minute. 



