RESPIRATION. 177 



The mucous membrane consists 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 cells (Fig. 147), 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 membrane. Beneath 

 these are one or more layers of more irregularly shaped cells. In the 

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

 nective-tissue corpuscles and capillary blood-vessels. 



Numerous mucous glands are situate on the exterior and in the 

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

 ting the various structures which form the wall of the trachea, and. 

 opening 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. 145), resemble 

 the trachea exactly in structure, and in the arrangement of their carti- 

 laginous rings. On entering the substance of the lungs, however, the; 

 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 dis- 

 tributed 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 subdivisions 

 of the lungs, called lobules. 



All the larger branches still have walls formed of tough membrane, 

 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 (Fig. 

 148). The mucous membrane is abundantly provided with mucous 

 glands. 



As the bronchi 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. And when the bronchi, by successive branches, are 

 reduced to about ^ of an inch in diameter, they lose their cartilaginous 

 element altogether, and their walls are formed only of a tough fibrous 

 elastic membrane, with circular muscular fibres; they are still lined, how- 

 ever, 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 almost cubical. In the smaller bronchi the circular muscular 

 VOL. I. 12. 



