RESPIRATION". 255 



brane in which they are closed (fig. 202). The cartilages of the trachea 

 and bronchial tubes are of the hyaline variety. 



Immediately within this tube, at the back, is a layer of unstriped 

 muscular fibres, which extends, transversely, between the ends of the 

 cartilaginous rings to which they arc attached, and opposite the inter- 

 vals between them, also; their evident function being to diminish, when 

 required, the calibre of the trachea by approximating the ends of the 

 cartilages. Outside there are a few longitudinal bundles of muscular 

 tissue, which, like the preceding, are attached both to the fibrous and 

 cartilaginous framework. 



The mucous membrane consists to a great extent of adenoid tissue, 

 separated from the stratified columnar epithelium which lines it by a 

 homogeneous basement membrane. This is penetrated here and there 

 by channels which connect the adenoid tissue of the mucosa with the 

 intercellular substance of the epithelium. The stratified columnar 

 epithelium is formed of several layers, of which the most superficial layer 

 is ciliated, and is often branched downward to join connective tissue 

 corpuscles; while between these branched cells are smaller elongated 

 cells prolonged up toward the surface and down to the basement mem- 

 brane. Beneath these are one or more layers of more irregularly-shaped 

 cells. Many of the superficial cells are of the goblet variety. In the 

 deeper part of the mucosa are many elastic fibres between which lie 

 connective-tissue corpuscles and capillary blood-vessels. 



Numerous mucous glands are situated on the exterior and in the 

 substance of the fibrous framework of the trachea; their ducts perforat- 

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

 ing through the mucous membrane into the interior. 



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

 is shorter, broader, and more horizontal than the left (fig. 200), resem- 

 ble the trachea exactly 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, 

 into a number of smaller and smaller branches, which penetrate into 

 every part of the organ, until at length they end in the smaller sub- 

 divisions of the lungs, called lobules. 



All the larger branches have walls formed of tough membrane, con- 

 taining portions of cartilaginous rings, by which they are held open, and 

 unstriped muscular fibres, as well as longitudinal bundles of elastic tis- 

 sue. They are lined by mucous membrane, the surface of which, like 



