136 



RESPIRATION 



lined by ciliated epithelium, and has mucous glands in its 

 substance (Figs. 47, 48). 



The Bronchi. The primitive bronchi are of the same es- 

 sential structure as the trachea. The right is the larger, 

 shorter, and more nearly horizontal. This probably ac- 

 counts for the more frequent lesions in the right lung. Pen- 



BrorteMal Musc/e. 



Bronchial ' flrtery. 



G/anc/ acini & cfucf. 



FIG. 49. 



B, intra-pulmonary bronchus of cat; P. A. and P.V., pulmonary artery and 

 vein; bv, bronchial vein; V ', air vesicles. (Stirling.) 



etrating the lung substance they divide and subdivide until 

 each, by its ramifications, communicates with every air vesi- 

 cle in that lung. When the primitive bronchus has divided, the 

 incomplete cartilaginous rings are replaced by irregular 

 plates of cartilage, which are so arranged as to completely 

 encircle the tube. These extend as far as the division of 

 the tubes into branches %o in. in diameter. 



Surrounding the tubes in the lung substance is a circular 

 layer of plain muscular fibers; these cease only at the air 



