246 



RESPIRATION 



cellular substance of the epithelium. The stratified columnar epithelium 

 is formed of several layers, of which the most superficial layer is ciliated and 

 the cells often branched downward. Many of the superficial cells are of the 

 goblet variety. In the deeper part of the mucosa are many elastic fibers 

 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 perforating the various 



FIG. 219. Section of the Trachea, a, Columnar ciliated epithelium; bandc, proper structure 

 of the mucous membrane, containing elastic fibers cut across transversely; d, submucous tissue 

 containing mucous glands, e, separated from the hyaline cartilage, g, by a fine fibrous tissue, /, 

 h, external investment of fine fibrous tissue. (S. K. Alcock.) 



structures which form the wall of the trachea, and opening through the 

 mucous membrane into the cavity of the trachea. 



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 un- 

 striped muscle arranged circularly beneath the mucous membrane, forming 

 the, muscularis mucosa. On entering the substance of the lungs the carti- 

 laginous rings, although they still form only larger or smaller segments of 



