174 NORMAL HISTOLOGY. 



projecting rings, which bound the opening. From 

 these rings of elastic fibres which surround the 

 openings into the alveoli, other elastic fibres are 

 given off, which, dividing and subdividing, stretch 

 over the walls of the, air-vessels in the form of a 

 wide-meshed net, the spaces between the fibres be- 

 ing occupied by an extremely thin, structureless 

 membrane, in which lies an occasional oval nucleus. 

 The alveoli in the adult are lined with a single 

 layer of flattened, polygonal, epithelial cells. These 

 are of two kinds : first, small granular, nucleated 

 cells ; and second, cells which are larger, more ir- 

 regular in form, very thin and transparent, and 

 usually without nuclei. The relative proportion of 

 these two kinds of cells is variable, and their out- 

 lines, especially those of the larger cells, it is difficult 

 to see distinctly without resorting to silver-staining. 

 These thin transparent cells which partially line the 

 air-vesicles are sometimes called respiratory epithe- 

 lium, and it was formerly supposed that such cells 

 were confined to the terminal air-spaces. It has 

 been recently shown, however, chiefly by the re- 

 searches of Kolliker, that the respiratory epithelium 

 is abundant in the smaller bronchi as well, and these 

 he accordingly calls respiratory bronchioles. The 

 peculiar character and distribution of this epithe- 

 lium, which seems so well fitted to facilitate the 

 interchange of material between air and blood in the 

 lungs, would seem to indicate, therefore, that the 



