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HAND-BOOK OF PHYSIOLOGY 



fibres are more abundant than in the trachea and larger bronchi, and form 

 a distinct circular coat. 



The Lungs and Pleura. 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 



FIG. 148. Transverse section of a bronchus, about one-fourth of an inch in diameter, e, Epithe- 

 lium (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; gr, mucous gland. (F. E. Schulze.) 



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 

 pleura, one layer of which adheres closely to the surface of the lung, 



FIG. 149. Transverse section of the chest (after Gray). 



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 mem- 

 branes, will be best understood by reference to Fig. 149. The appearance 



