THE RESPIRATORY SYSTEM 317 



ciliated epithelium, resting upon a basement membrane. Beneath 

 this is a richly elastic stroma containing bundles of circularly dis- 

 posed smooth muscle cells. The epithelium becomes gradually lower 

 and non-ciliated, and near the distal end of the terminal bronchus 

 there appear small groups or islands of flat, non-nucleated epithelial 

 cells— respiratory epithelium. 



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Fig. 215. — Section of Lung of Rat to show Arrangement of Bronchial Ramifications and 

 of Alveoli within a Single Lobule. W, Lobular or sublobular bronchus; &zY, intralobular 

 bronchus; ba, terminal bronchus; ;•, dilatation sometimes called vestibule; ca, alveolar 

 passage; i, portion sometimes called infundibulum; d, alveoU, some of which are so cut 

 as to show their openings into the infundibulum and alveolar canals, etc., while others 

 appear closed. X60. (Prenant.) 



The alveolar passage (Fig. 215, ca). Here the cuboidal epithelium 

 is almost completely replaced by the respiratory. Beneath the 

 epithelium the walls have a structure similar to those of the distal 

 end of the terminal bronchus, consisting of delicate fibro-elastic tissue 

 with scattered smooth muscle cells. The basement membrane is 

 extremely thin. 



