STRUCTURE OF THE LUNGS. 199 



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 they might be adherent completely, one 

 to the other, 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.* 



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 

 substance of the organ, divide and subdivide into a number 

 of smaller and smaller branches, which penetrate to every 

 part of the organ, until at length they end in the smaller 

 subdivisions of the lung called lobules. All the larger 

 branches have walls formed of tough membrane, contain- 

 ing portions of cartilaginous rings, by which they are held 

 open, and unstriped muscular fibres, as well as longi- 

 tudinal bundles of elastic tissue. They are lined by 

 mucous membrane, the surface of which, like that of the 

 larynx and trachea, is covered with vibratile ciliary epi- 

 thelium (fig. 58). 



* It may be here 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 (fig. 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. 



