BRONCHIAL TUBES 237 



In such tubes typical bronchi the wall, as in the trachea, com- 

 prises : 



1. A mucous membrane. 



2. A submucosa. 



3. A fibro-cartilaginous coat. 



The mucous membrane, a continuation of that of the trachea, is 

 lined by tall, columnar, pseudo-strati fied, ciliated epithelium which 

 rests upon a distinct elastic basement membrane. The epithelium 

 is thrown into wavy longitudinal folds. The tunica propria is 

 extremely vascular ; it possesses an abundant supply of thin-walled 

 veins of small caliber, together with many lymphatic vessels. Its 

 connective tissue forms a delicate fibrous reticulum in the meshes 

 of which are many lymphatic corpuscles. The outer portion of 

 the tunica propria contains bundles of fine longitudinal elastic 

 fibres, which form a complete layer about the tube. This elastic 

 layer is thickest opposite the ridges and thinnest opposite the 

 troughs of the epithelial waves. 



The outer boundary of the mucous membrane contains a well- 

 developed muscular is mucosce composed of interlacing bundles of 

 circular smooth muscle fibres. This layer forms a complete mus- 

 cular coat which is here and there pierced by the ducts of mucous 

 glands whose secreting portions lie in the submucosa. 



The submucosa, by its broad-meshed areolar tissue, loosely 

 unites the mucous membrane to the cartilage plates. This coat 

 contains the larger blood vessels, nerves, and lymphatics which are 

 distributed to the mucosa. It also contains the secreting portions 

 of many tubulo-acinar mucous glands, which occur in groups that 

 in the larger bronchi almost completely surround the tube. The 

 number and size of these glands is in direct proportion to the size 

 of the bronchus. The efferent ducts of the mucous glands pene- 

 trate the muscularis mucosae and open upon the free surface 

 in the interval between adjacent folds of the epithelial lining. In 

 the tunica propria the ducts possess ampullary dilatations which 

 are lined by ciliated cells and contain portions of the mucous 

 secretion. 



The nbro-cartilaginous coat is formed by a dense fibrous mem- 

 brane in which the cartilages are embedded. The plates of hya- 

 line cartilage vary much in number and size, being more or less 

 highly developed in proportion to the size of the bronchial tube. 

 They possess at all times a somewhat crescentic shape. In the 

 larger bronchi three or four cartilage plates with tfveflapping 



