THE RESPIRATORY SYSTEM, 65 



inspiration; that which drives the air from the 

 lungs, expiration. There are quite a number of 

 muscles concerned in these acts. When the lungs 

 are to be filled, these muscles expand the walls of 

 the chest so as to enlarge the space inside. 

 The air rushes in through the mouth, nose and wind- 

 pipe, and fills % the cells. Then a reverse action is 

 produced by the muscles. The chest contracts and 

 the air is forced out from the lungs by the same way 

 through which it entered, but robbed of its oxygen 

 and mixed with gases discharged from the body. 



Between the chest and abdomen is a 

 iiTsM "scies! 1 " broad partition muscle called the dia- 

 phragm. This muscle is chiefly con- 

 cerned in ordinary, gentle breathing. When the 

 lungs are to be filled, the diaphragm moves down- 

 ward, pressing upon the contents of the abdomen 

 and enlarging the cavity of the chest, giving the 

 lungs room for full expansion, provided the act of 

 inspiration is unrestricted and complete. In expira- 

 tion, the diaphragm rises and diminishes the capac- 

 ity of the chest, forcing the air out of the lungs. In 

 forced breathing, the muscles between the ribs, 

 called the inter-cosfal muscles (6, rib section], take a 

 prominent part. These and other muscles also take 

 more or less part in ordinary breathing. 



The inside of the chest is lined with a 



delicate web called the pleura. This is 



also spread as a covering over the lungs. It secretes a 



OK THJt 



UNIVERSITY 



