RESPIRATION. 173 



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 epithelium (Fig. 

 146). 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 in diameter, they lose their cartilagi- 

 nous 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 epithelium, th& 

 length of the cells bearing the cilia having become so far diminished 

 that the cells are now almost cubical. In the smaller bronchi the circu- 



FIG, 148. Transverse section of the chest. 



lar muscular fibres are more abundant than in the trachea and 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 ap- 

 pear 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. 



The Pleura. Each lung is enveloped by a serous membrane the 

 vleura, one layer of which adheres closely to the surface, and provides it 

 with its smooth and slippery covering, while the other adheres to the in- 

 ner surface of the chest-wall. The continuity of the two layers, which 



