136 RESPIRATION 



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

 tial structure as the trachea. The right is the larger, shorter, 

 and more nearly horizontal. This probably accounts for the 

 more frequent lesions in the right lung. Penetrating the lung 

 substance they divide and subdivide until each, by its ramifica- 

 tions, communicates with every air vesicle in that lung. When 

 the primitive bronchus has divided, the incomplete cartilaginous 



Bronchial Muscfe. 



Bronchia/ flrtery. 



G fa not acini fe of net 



FIG. 49. 



T. S., intra-pulmonary bronchus of cat. PA and PV, pulmonary artery and vein; 

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



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 -^ in. in diameter. 



Surrounding the tubes in the lung substance is a circular layer 

 of plain muscular fibers; these cease only at the air vesicles. 

 Elastic fibrous tissue is also present everywhere in the bronchial 

 walls and is continued over the vesicles themselves. 



Bronchial tubes above - in. in diameter have in their walls 



