GLANDS. 95 



tubes chminish in calibre, and finally, when this has 

 reached a diameter of one-half of a line, they are no 

 longer to be found. Tubes of this size, in fact, consist 



O I ' 



simply of an extremely delicate mucous membrane, 

 lined externally by scattered muscular fibres, and 

 within, by ciliated epithelium. 



Clusters of gland vesicles are found imbedded in Glands. 

 the thickness of the walls of the trachea and bronchi. 

 Very numerous in the commencing trunks of the bron- 

 chial tree, they become less and less frequent as its 

 branches diminish in size, and, according to Kolliker, 

 are no longer to be seen in bronchise of from one to 

 one and a half lines in diameter. These little glands, 

 which are hardly one-fourth of a line in diameter, are 

 to be found in the deepest portion of the mucous coat 

 of the tube, or rather lying upon the inner surface of 

 its fibrous coat. Their epithelial cells are polygonal, 

 whilst those lining their ducts, which open into the 

 bronchial tubes, are cylindrical, but not ciliated.* 



The pleura, like the other serous membranes, are Pleur *- 

 not very complicated in their structure. They are 

 made up of a somewhat dense interlacement of fibres 

 connective and elastic and, upon the free surface 

 of the membrane thus formed, a simple layer of pave- 

 ment epitheliunl. They receive numerous blood- 



* Waters (op. cit. p. 122) describes two sets of glands as belonging to 

 the mucous membrane of the trachea and bronchial tubes : one consisting 

 of simple mucous follicles, found everywhere on the surface of the mem- 

 brane ; the other, larger in size and compound in character, found only 

 in the posterior, or membranous portion of the trachea and larger bron- 

 chial ramifications. It is these latter which are supposed to furnish the 

 very tenacious and viscid masses of sputa so characteristic of certain 

 stages of bronchitis. (Ed.) 



