1 88 RESPIEATIOX. 



It lias 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 it. This enveloping bag, which is 

 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 

 speak, to the inner surface of the walls and floor of the 

 chest. 



This 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 110 separation, in the slightest degree, of one 

 from the other.* 



The trachea, or tube through which air passes to the 

 lungs, divides into two branches one for each lung ; 

 -and these primary branches, or bronchi, after entering the 



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

 .really continuous with the inner smooth lining of the walls and floor of 

 the chest, as will be readily seen in fig. 56. Hence the membrane 

 which covers the lung is called the visceral layer of the pleura, and that 

 which lines the walls and floor of the chest the parietal layer. The 

 appearance of a cavity or space (tig. 56) between the visceral layer of 

 pleura (covering the lungs) and the parietal layer (covering the inner 

 surface of the wall of the chest and upper part of the diaphragm) is 

 only inserted for the sake of distinctness. 



