346 RESPIRATION [CH. XXIV. 



entering the substance of the lungs the cartilaginous rings, although 

 they still form only larger or smaller segments of a circle, are no 

 longer confined to the front and sides of the tubes, but are distributed 

 impartially to all parts of their circumference. 



The bronchi* divide and subdivide, in the substance of the lungs, 

 t/ J into a number of smaller and smaller branches (bronchial tubes), 

 which penetrate into every part of the organ, until at length they 

 end in the smaller subdivisions of the lungs called lobules. 



All the larger branches have walls formed of fibrous tissue, con- 

 taining portions of cartilaginous rings, by which they are held open, 

 and unstriped muscular fibres, as well as longitudinal bundles of 

 elastic tissue. They are lined by mucous membrane the surface of 

 which, like that of the larynx and trachea, is covered with ciliated 



FIG. 320. Transverse section of a bronchial tube, about inch in diameter, e, Epithelium (ciliated); 

 immediately beneath it is the corium of the mucous membrane, of varying thickness ; m, muscular 

 layer; s.m, submucous tissue; /, fibrous tissue; c, cartilage enclosed within the layers of fibrous 

 tissue; g, mucous gland. (F. B. Schulze.) 



epithelium, but the several layers become less and less distinct until 

 the lining consists of a single layer of short columnar cells covered 

 with cilia (fig. 320). The mucous membrane is abundantly provided 

 with mucous glands. 



As the subdivisions become smaller and smaller, and their walls 

 thinner, the cartilaginous rings become scarcer and more irregular, 

 until, in the smaller bronchial tubes, they are represented only by 

 minute and scattered cartilaginous flakes. When the bronchial tubes, 

 by successive branchings, are reduced to about T V of an inch ('6 mm.) 

 in diameter they lose their cartilaginous element altogether, and their 

 walls are formed only of a fibrous elastic membrane with circular 

 muscular fibres; they are still lined, however, by a thin mucous 

 membrane with ciliated epithelium, the length of the cells bearing 

 the cilia having become so far diminished that the cells are now 

 cubical. In the smaller bronchial tubes the muscular fibres are 



