348 



RESPIRATION 



[CH. XXVI. 



an inch to an inch in diameter, and is essentially a tube of fibro-elastic 

 membrane, within the layers of which are imbedded a series of carti- 

 laginous rings, from sixteen to twenty in number. These rings ex- 

 tend only around the front and sides of the trachea (about two-thirds 

 of its circumference) and are deficient behind; the interval between 

 their posterior extremities is bridged over by a continuation of the 

 fibrous membrane in which they are enclosed (fig. 28 SB). The inner 

 surface of the trachea is lined with ciliated epithelium; this, 



together with the basement mem- 

 brane on which it rests, and a 

 deeper layer of connective tissue, 

 forms the mucous membrane of 

 the trachea. 



Numerous mucous glands are 

 situated in the substance of the 

 mucous membrane ; their ducts 

 perforate the various structures 

 which form the wall of the trachea, 

 and open through the mucous mem- 

 brane into the interior. A layer 

 of unstriped muscle is situated 

 beneath the mucous membrane 

 at the back of the tube where 

 the cartilaginous rings are absent. 

 The two bronchi into which the trachea divides, 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. 



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

 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, con- 

 taining 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 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. 285). 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 irregular, 

 until, in the smaller bronchial tubes, they are represented only by 

 minute and scattered cartilaginous flakes. When the bronchial tubes, 



FIG. 284. Ciliated epithelium of the human 

 trachea, a, Layer of longitudinally arranged 

 elastic fibres ; b, basement membrane ; 

 c, deepest cells circular in form ; d, inter, 

 mediate elongated cells ; e, outermost layer 

 of cells fully developed and bearing cilia, 

 x 350. (Kolliker.) 



