222 ELEMENTS OF HISTOLOGY. [Chap. xxvni. 



or mucous glands in their wall. This is made up of 

 three coats : (a) a tiny epithelium a single layer of 

 small polyhedral granular-looking cells ; (6) a circular 

 muscular coat of non-striped muscular tissue; and 

 (c) a fine adventitia of elastic fibres, arranged chiefly 

 as longitudinal networks. 



292. Tracing the elements constituting the wall 

 of a terminal bronchiole into the infundibula and air- 

 cells (Fig. 128), we find the following changes : (a) the 

 polyhedral granular-looking epithelial cells forming a 

 continuous lining in the terminal bronchiole, are trace- 

 able into the iiifundibulum only as larger or smaller 

 groups ; between these groups of small polyhedral 

 granular-looking cells large, flat, transparent, homo- 

 geneous, nucleated, epithelial scales make their ap- 

 pearance. The farther away from the terminal 

 bronchiole, the fewer are the groups of polyhedral 

 granular-looking cells. In all infundibula, however, 

 the transparent scales form the chief lining. This 

 becomes still more marked in the air-cells. There 

 the small polyhedral granular-looking cells are trace- 

 able only singly, or in groups of two or three (Elens), 

 the rest of the cavity of the air-cells being lined with 

 the large transparent scales. 



In the foetal state all cells lining the infundibula 

 and air-cells are of the small polyhedral granular- 

 looking variety (Kiittner). With the expansion of 

 the lungs during the first inspiration many of these 

 cells change into the large transparent scales, in order 

 to make up for the increment of surface. A lung 

 expanded ad maximum shows much fewer or none of 

 the small polyhedral cells; while a lung that is col- 

 lapsed shows them in groups in the infundibula, 

 and isolated or in twos or threes in the alveoli. 



293. (b) The circular coat of non-striped muscular 

 tissue of the terminal bronchiole is continued as a con- 

 tinuous circular coat but slightly thinner on the 



