CH. XXIV.] THE TRACHEA. 34 1 



various structures which form the wall of the trachea, and open 

 through the mucous membrane into the interior (fig. 310). 



The two bronchi into which the trachea divides, of which the 

 right is shorter, broader, and more horizontal than the left 

 (fig. 308), resemble the trachea in structure, with the difference 

 that in them there is a distinct layer of unstriped muscle 

 arranged circularly beneath the mucous membrane, forming the 

 muscularis mucosce. On entering the substance of the lungs 

 the cartilaginous rings, although they still form only larger or 

 smaller segments of a circle, are no longer confined to the front 

 and sides of the tubes, but are distributed impartially to all parts 

 of their circumference. 



The bronchi divide and subdivide, in the substance of the lungs, 



g.in 



Fig. 311. Transverse section of a bronchial tube, about J inch in diameter. <?, epithelium 

 (ciliated) ; immediately beneath it is the eorium of the mucous membrane, of van-ing 1 

 tliii-kin- ; nt, muscular layer ; >./. nubmucouH tissue ; '. fibrous tissue ; e, cartilage 

 enclosed within the layers of fibrous tissue ; '/. mucous gland. (F. . Schulze.) 



into a number of smaller and smaller branches (bronchial tubes), 

 which penetrate into every part of the organ, until at length they 

 end in the smaller subdivisions of the lungs called lobules. 



All the larger branches have walls formed of fibrous tissue, 

 containing portions of cartilaginous rings, by which they are held 

 open, 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, but the several layers become 

 less and less distinct until the lining consists of a single layer of 

 short columnar cells covered with cilia (fig. 311). The mucous 

 membrane is abundantly provided with mucous glands. 



As the subdivisions become smaller and smaller, and their walls 

 thinner, the cartilaginous rings become scarcer and more irregu- 

 lar, until, in the smaller bronchial tubes, they are represented 



