THE RESPIRATORY TRACT. 



713 



The lining epithelium is of the ciliated columnar variety, 

 indistinctly stratified the cilia being directed, as in the larynx r 

 toward the throat. Its prolongations form racemose mucous 

 glands, lying in flat layers close above the perichondrium, and 

 globular formations in the posterior wall j in the latter situation 

 they are especially large and numerous. Their ducts traverse the 

 mucosa in an oblique direction, and are lined by columnar epi- 

 thelia, which, near the opening at the surface, are supplied with 

 cilia. Similar features can also be observed in the bronchi. 



(4) The lungs are composed of minute cells or alveoli, which 

 are inclosed by fibrous connective tissue, a group of alveoli being 

 connected with a bronchiolus or alveolar duct. This union of 

 alveoli and alveolar duct is called a lobule, the general shape of 

 which is conical, with the base directed toward the periphery of 



FIG. 317. TRACHEA OF A CHILD. TRANSVERSE SECTION. 



E, ciliated columnar epithelium ; 8, hyaline or basement-layer ; F, fibrous connective tissue 

 of the mucosa ; M, circular bundles of smooth muscle-fibers ; I), D, ducts of mucous glands, in 

 longitudinal and transverse section ; G, racemose mucous glands ; P, perichondrium ; C, hya- 

 line cartilage. Magnified 200 diameters. 



the lung, and its apex connected with a bronchiolus. This rela- 

 tion is most marked in the peripheral portions of the lungs. 

 The terminal alveoli bear the superfluous name " infundibulum. 77 

 The widening of the bronchioli into the lung-cells is abrupt, and 

 single cells, or cells arranged in small groups, may be seen 

 attached to the bronchiolus before it terminates into the lobule. 

 The lobules are separated from each other by a broader layer of 



