CH. XXVI.] THE LUNGS AND PLEUILE 349 



by successive branchings, 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 fibrous elastic membrane with circular 

 muscular fibres; 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 now 

 cubical. In the smaller bronchial tubes the muscular fibres are 

 relatively more abundant than in the larger ones, and form a 

 distinct circular coat. 



Most of the structures which have been described are of some 

 clinical importance. The secretion of the mucous glands, for 

 instance, may be greatly increased in the condition known as catarrh 

 of the mucous membrane. The secretion, or phlegm, is worked up to 



s.rn 



FIG. 285. Transverse section of a bronchial tube, about inch in diameter, e, Epithelium (ciliated), 

 immediately beneath it is the corium of the mucous membrane, of varying thickness ; m, muscular 

 layer; s.w, submucous tissue; /, fibrous tissue; c, cartilage enclosed within the layers of fibrous 

 tissue ; g, mucous glands. (F. E. Schulze.) 



the larynx by the ciliated epithelium. Its presence irritates the 

 very sensitive surface of that organ, and induces a cough by which 

 the phlegm is expelled from the respiratory passages into the 

 mouth. 



The whole inner surface of the bronchi may become inflamed 

 and filled with fluid, through which the air has to be forced at each 

 respiration (bronchitis). 



A disorder of another nature, bronchial asthma, is caused by 

 undue contraction of the circular muscles of the bronchi. The 

 passages are thus rendered too narrow for the necessary volume of 

 air to pass conveniently, and as a result the respiration becomes 

 forced. The bronchial muscles are supplied by the vagus nerve, and 

 relaxation of them may be brought about by drugs which prevent 

 the passage of impulses along the vagus. 



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



