198 RESPIRATION. 



air-tubes may have been cut across, and show, on their 

 surface of the section, their tubular structure. 



In fact, however, the lungs are hollow organs, and we 

 may consider them as really two bags containing air, each 

 of which communicates by a separate orifice with a common 

 air-tube (fig. 31), through the upper portion of which, 

 the larynx, they freely communicate with the external 

 atmosphere. The orifice of the larynx is guarded by 

 muscles, and can be opened or closed at will. 



It has been said, in the preceding chapter, that each 

 lung is enveloped in a distinct fibrous bag, with a smooth, 

 slippery lining, and that the outer smooth surface of 

 the lung glides easily on the inner smooth surface of the 

 bag which envelopes it. This enveloping bag, which is 



Fig. 56.* 



called the pleura, is easily seen in the dead subject ; and 

 when it is opened, as in an ordinary post-mortem examina- 

 tion, there is a considerable space left, by the elastic recoil 

 of the lung, between the outer surface of the lung and the 

 inner surface of the pleura, which is left sticking, so to 



Fig. 56. Transverse section of the chest (after Gray). 



