282 RESPIRATION 



the muscularis mucosa. On entering the substance of the lungs the carti- 

 laginous 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 into 

 smaller and smaller branches, 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 tough membrane, contain- 

 ing portions of cartilaginous rings, by which they are held open, and un- 

 striped muscular fibers, 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 epithelium; but the several 

 layers become less and less distinct until the lining consists of a single layer 

 of more or less cubical cells covered with cilia, figure 221. The mucous 

 membrane is abundantly provided with mucous glands. 



As the bronchi become smaller and smaller and their walls thinner, the 

 cartilaginous rings become fewer and more irregular, until in the smaller 

 bronchial tubes they are represented only by minute and scattered cartilag- 

 inous flakes. And when the bronchi by successive branches are reduced 

 to about -fa of an inch, 0.6 mm., in diameter, they lose their cartilaginous ele- 

 ment altogether and their walls are formed only of a tough, fibrous, elastic 

 membrane with circular muscular fibers. 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 

 almost cubical. In the smaller bronchi the circular muscular fibers are 

 relatively more abundant than in the larger bronchi and form a distinct 

 circular coat. 



The Lungs and Pleurae. The lungs occupy the greater portion of 

 the thorax. They are of a spongy elastic texture, and on section appear 

 to the naked eye as if they were in great part solid organs, except where 

 branches of the open bronchi or air-tubes may have been cut across and show 

 on the surface of the section. In fact, however, the lungs are hollow organs 

 composed of a mass of air cavities all of which communicate finally with 

 the common air-tube, the trachea. 



Each lung is enveloped by a serous membrane, the pleura, which ad- 

 heres closely to its surface and provides it with its smooth and slippery 

 covering. This same membrane lines the inner surface of the chest wall. 

 The continuity of this membrane, which forms a closed sac as in the case 

 of other serous membranes, will be best understood by reference to figure 222. 

 The appearance of a space, however, between the pleura which covers the 

 lung, visceral layer, and that which lines the inner surface of the chest, parietal 

 layer, is inserted in the drawing only for the sake of distinctness. These 



