256 HANDBOOK OF PHYSIOLOGY. 



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 (fig. 203). 

 The mucous membrane is abundantly provided with mucous glands. 



As the bronchi 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 scat- 

 tered cartilaginous flakes. And when the bronchi, by successive branches 

 are reduced to about ^ of an inch (.6 mm.) in diameter, they lose their 

 cartilaginous element altogether, and their walls are formed only of a 

 tough fibrous elastic membrane, with circular muscular fibres; they are 

 still lined, however, by a thin mucous membrane, with ciliated epithe- 

 lium, the length of the cells bearing the cilia having become so far 

 diminished that the cells are now almost cubical. In the smaller bron- 



Fig. 203. Transverse section of a bronchus, about ^ inch in diameter, e. Epithelium (ciliated) ; 

 immediately beneath it is the mucous membrane or internal fibrous layer, of varying thickness; m, 

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

 of fibrous tissue ; g, mucous glaud. (F. E. Schulze.) 



chi the circular muscular fibres 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 here 

 and there, at certain points, where branches of the bronchi or air-tubes 

 may have been cut across, and show, on the surface of the section, their 

 tubular structure. In fact, however, the lungs are hollow organs, each 

 of which communicates by a separate orifice with a common air-tube, 

 the trachea. 



Each lung is enveloped by a serous membrane the pleura, one layer 

 of which adheres closely to its surface, and provides it with its smooth 

 and slippery covering, while the other adheres to the inner surface of 

 the chest-wall. The continuity of the two layers, which form a closed 

 sac, as in the case of other serous membranes, will be best understood 

 by reference to fig. 204. The appearance of a space, however, between 



