172 HANDBOOK OF PHYSIOLOGY. 



The mucous membrane consists of adenoid tissue, separated from the 

 stratified columnar epithelium which lines it by a homogeneous base- 

 ment 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. 146), of which the most superficial layer is 

 ciliated, and is often branched downwards to join connective-tissue cor- 

 puscles, while between these branched cells are smaller elongated cells 

 prolonged up towards 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 connective-tissue corpuscles and capillary blood-vessels. 



Numerous mucous glands are situate on the exterior and in the sub- 

 stance of the fibrous framework of the trachea ; their ducts perforating 



FIG 147. Transverse section of a bronchus, about y f inch in diameter, e, Epithelium (ciliated^; 

 immediately beneath il is the mucous membrane or internal fibrous layer, of varying thickness; m, 

 muscular layer; s, m, submucous tissue; /, fibrous tissue; c, cartilage inclosed within the layers of 

 fibrous tissue; g, mucous gland. (F. E. Schultze.) 



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. 144), resem- 

 ble the trachea exactly in structure, and in the arrangement of their car- 

 tilaginous 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 

 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 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, 



