THE RESPIRATORY ORGANS. 



251 



-*:'% ^ 



Section of portion of bronchus of child : a, epithelium ; b, 

 basement-membrane ; c, stroma of mucosa ; d, layer of in- 

 voluntary muscle ; e t submucosa ; _/", acini of mucous glands ; 

 h, blood-vessels ; *, obliquely cut duct of mucous glands. 



noted ; when the diameter of the twig no longer exceeds one milli- 

 metre the tube is termed 

 a terminal bronchus. FIG. 286. 



According to Miller, 

 the distal extremity of 

 the terminal bronchus 

 communicates with sev- 

 eral spherical cavities, 

 the atria. Each of the 

 latter in torn opens into 

 a group of larger and 

 less regular spaces, the 

 air-sacs, into which di- 

 rectly open the ultimate 

 air-spaces, the alveoli 

 or air-cells. The latter 

 open not only into the 

 air-sacs and atria, but 

 also beset the alveolar 

 ducts as minute saccula- 

 tions that impart an ir- 

 regular contour to the air-tube. 



The walls of the terminal bronchial tubes consist at first of a 

 single layer of ciliated columnar epithelium, outside of which the 

 mucosa contains longitudinal elastic fibres 

 and thin, irregular, annular bundles of non- 

 striped muscle. The mucous glands and 

 the cartilaginous plates are wanting in the 

 terminal tubes. Within the latter the ciliated 

 cells disappear, a simple columnar epithelium 

 existing for some distance, which, in turn, is 

 replaced by cuboidal cells on approaching the 

 atria. 



The walls of the irregular atria suffer still 

 further reduction, the fibrous coat becoming 

 greatly thinned, while the mucosa is reduced to panments of air-passage : B, 



... . c ri i . . terminal bronchus; A, atrium; 



a delicate tunica propna of fibro-elastic tissue, 5. >air . saC)intowhichopenair . 



in which bundles of non-striped muscle may ceils (C). Branches of pulmo- 

 nary artery follow bronchus ; 



remain. those of pu i monary vein i ie at 



The epithelium of the smaller passage, at periphery. (Miller.-) 

 first low cuboidal in character, rapidly assumes 



a flat polygonal type ; towards the air-sacs large flat plates appear 

 among the smaller polygonal cells, and become more numerous as 

 these terminal divisions of the air-passages are neared. 



FIG. 287. 



Diagram of terminal com- 



