THE TRACHEA AND BRCNCHI 



47 



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 

 smaller and smaller branches, which penetrate into every part of the organ 



FIG. 220. Ciliary Epithelium of the Human Trachea, a. Layer of longitudinally arranged 

 elastic fibers; b, basement membrane; c, deepest cells, circular in form; d, intermediate elongated 

 cells; e, outermost layer of cells fully developed and bearing cilia. X 350. (Kolliker.) 



until at length they end in the smaller subdivisions of the lungs called 

 lobules. 



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

 ing portions of cartilaginous rings, by which they are held open, and un- 

 striped muscular fibers, as well as longitudinal bundles of elastic tissue. 



FIG. 22i. Transverse Section of a Bronchus, about $ inch in Diameter, e, Epithelium 

 (ciliated); immediately beneath it is the mucous membrane or internal fibrous layer, of varying 

 thickness; m, muscular layer; 5. m, submucous tissue; /, fibrous tissue ; c, cartilage enclosed within 

 the layers of fibrous tissue; g, mucous gland. (F. E. Schulze.) 



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 more or less cubical cells covered with cilia, figure 221. The mucous 

 membrane is abundantly provided with mucous glands. 



