300 



HISTOLOGY 



replaced by irregular plates found on all sides of the tube (Fig. 295). 

 These diminish in size as the bronchi become smaller, and disappear in 

 those about i mm. in diameter. Usually the cartilages are hyaline, but 

 elastic cartilage is said to occur in places. The circular muscle fibers 

 form a layer completely surrounding the tube internal to the cartilages. 

 Branched tubulo-alveolar bronchial glands extend further down the tubes 

 than the cartilages. In the larger bronchi they are present in great numbers, 



Tunica Circular ?"':; 



Epithelium. propria. muscle fibers. : . /' : '' ; " 



Alveoli. 



Fat cells. 



Cartilage. 



Connective tissue. 

 Bronchial gland. 



Duct of gland. 



FIG. 295- CROSS SECTION OF A BRONCHUS 2 MM. IN DIAMETER, FROM A CHILD. 



and their bodies lie outside of the muscular layer and project into the spaces 

 between the cartilages. The mucosa is thrown into longitudinal folds; 

 it is covered with ciliated epithelium containing goblet cells and resembling 

 that of the trachea. Lymphocytes are numerous in the tunica propria, 

 sometimes collecting in solitary nodules and wandering into the epithelium. 

 The small bronchi, 0.5-1.0 mm. in diameter, are known as bronchioles. 

 They are free from cartilage and glands, and are lined throughout with 

 ciliated columnar epithelium. 



