48 RESPIRATION 



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 ^Q- of an inch, c.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 Plurae. 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 



FIG. 222. Transverse Section of the Chest. 



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 

 layers are, in health, everywhere in contact, one with the other; and between 

 them is only just as much fluid as will insure frictionless movement in their 

 expansion and contraction. 



