156 RESPIRATION. 



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 envelops 



FIG. 56. 



Transverse section of the chest (after Gray). 



it. This enveloping bag, which is called the pleura, is easily 

 seen in the dead subject ; and when it is opened, as in an ordi- 

 nary post-mortem examination, there is a considerable space 

 left, by the elastic recoil of the lung, between the outer sur- 

 face of the lung and the inner surface of the pleura, which is 

 left sticking, so to speak, to the inner surface of the walls and 

 floor of the chest. 



The space, however, between the lung and the pleura does 

 not exist (except in some cases of disease) so long as the chest 

 is not opened ; and, while considering the subject of normal 

 healthy respiration, we may discard altogether the notion of 

 any space or cavity between the lung and the wall of the 

 chest. So far as the movement of the lung is concerned it 

 might be adherent completely to the chest-wall, inasmuch as 

 they accompany each other in all their movements ; only there 

 is a slight gliding of the smooth surface of the lung on the 

 smooth inner surface of the pleura, but no separation, in the 

 slightest degree, of one from the other. 1 



1 It may be mentioned, that the smooth covering of the lung is 

 really continuous with the inner smooth lining of the walls arid 

 floor of the chost, as will be readily seen in Fig. 56. Hence the mem- 



