TERMINAL DIVISIONS OF BRONCHI AND ULTIMATE AIR-SPACES. 315 



and may even be found in the walls of the respiratory bronchioles. 

 The terminal bronchioles or alveolar ducts have an epithelium 

 which is of the cubic variety in their proximal portions, and which 

 changes to a squamous epithelium in their distal portions. 



The epithelium of the distal portions of the terminal bronchi- 

 oles or alveolar ducts, atria, and air-sacs (i i fj. to 15 p. in diameter) 

 and of the alveoli (the so-called respiratory epithelium) consists of 

 two varieties of cells (F. E. Schulze) smaller nucleated elements 

 and larger nonnucleated platelets (the latter derived very probably 

 from the former). The arrangement of the epithelial cells is gen- 

 erally such that the nonnucleated platelets rest directly upon the 

 blood capillaries, while nucleated cells lie between them. In am- 

 phibia the epithelium of the alveoli consists of cells, of which the 

 portion containing the nucleus forms a broad cylindric base ; from 



Nonnucleated epi- 

 thelial cell. 



Nucleated epithelial 



cell. 



Fig. 251. Inner surface of human alveolus treated with silver nitrate, showing respira- 

 tory epithelium ; X 2 4 (after Kolliker). 



the free end of each cell a lateral process extends over the adjoin- 

 ing capillary to meet a similar process from the neighboring cell. 

 When viewed from above, the basal portion of the cell appears 

 dark and granular, while the processes are clear and transparent. 

 These cells, together with their prolongations, are about 50 p. in 

 diameter. The surface view greatly resembles that of the human 

 respiratory epithelium (Duval, Oppel, 89). 



The terminal bronchioles or alveolar ducts have a distinct 

 layer of nonstriated muscle having annular thickenings about the 

 openings which lead to the atria. Muscular tissue is not found in 

 the walls of the atria, air-sacs, and air-cells or alveoli (Miller). 



Beneath the respiratory epithelium in the atria, air-sacs, and air- 

 cells, there is found a thin basement membrane, which is apparently 

 homogeneous. Here and there are found some fibrils of fibrous 



