356 



THE HUMAN BODY. 



atcd. are much shorter tnan the columnar superficial cell 

 layer of the larger tubes. The terminal alveoli (a, a, Fig. 

 106,) and the air-cells, #, which open into them, have walls 

 composed mainly of elastic tissue and lined by a single 



layer of flat, non-ciliated epi- 

 thelium, immediately beneath 

 which is a very close network 

 of capillary blood - vessels. 

 The air entering by the bron- 

 chial tube is thus only sepa- 

 rated from the blood by the 

 thin capillary wall and the 

 thin epithelium, both of 

 which are moist, and well 

 adapted to permit gaseous 

 diffusion. 



The Pleura. Each lung 

 is covered, except at one 

 point, by an elastic serous 

 membrane which adheres 

 tightly to it and is called the 

 pleura; that point at which 

 the pleura is wanting is called the root of the lung and is 

 on its inner side; it is there that its bronchus, blood-vessels 

 and nerves enter it. At the root of the lung the pleura 

 turns back and lines the inside of the chest cavity, as rep- 

 resented by the dotted line in the diagram Fig. 3. The 

 part of the pleura attached to each lung is its visceral, and 

 that attached to the chest-wall its parietal layer. Each 

 pleura thus forms a closed sac surrounding &pleural cavity, 

 in which, during health, there arc found a few drops of 

 lymph, keeping its surfaces moist. This lessens friction 

 between the two layers during the movements of the chest- 

 walls and the lungs; for although, to insure distinctness, 

 the visceral and parietal layers of the pleura are represented 

 in the diagram as not in contact, that is not the natural 

 condition of things; the lungs are in life distended so that 

 the visceral pleura rubs against the parietal, and the pleural 

 cavity is practically obliterated. This is due to the pressure 



FIG. 106. Two alveoli of the lung 

 highly magnified, b. 6, the air-cells, 

 or hollow protrusions of the alveolus, 

 opening into its central cavity ; c, ter- 

 minal branches of a bronchial tube. 



