312 



THE ORGANS 



calibre of the bronchi. It consists of loosely arranged connective 

 tissue. IMixed glands (Fig. 209) are present until a diameter of about 

 I mm. is reached, when they disappear. They lie in the submucosa 

 and frequently extend through between the cartilage plates into the 

 fibrous coat. The ducts pass through the muscular coat and open 

 into pit-like depressions lined with a continuation of the surface 

 ciliated epithelium. 



(5) The cartilages, which in the trachea and primary bronchi 

 form nearly complete rings, become gradually smaller, and finally 

 break up into short disconnected plates (Figs. 208 and 209). They 



Muscle 

 Epithelium Stroma coat 





Blood-vessel-^' )>"<,/■■ 7 //iv^>y 













Cartilage ^'' \ \ .|4'!f7,\V?>h 





^^5^' .;, 



Fibrous coat 

 Gla 





I 



■.4\ 



VJ 



A''\M 



Excretory duct 





Fig. 209. — Cross Section of Human Bronchus (of a child) of 2 mm. diameter. X30. 



(Stohr.) 



are frequently fibrocartilage rather than hyaline. These plates de- 

 crease in size and number, and are absent after a diameter of i mm. 

 is reached. Cartilage and mucous glands thus disappear at about the 

 same time, although it is common for glands to extend over into 

 smaller bronchi than do the cartilage plates. 



The bronchi down to a diameter of from 1.5 to i mm. are inter- 

 lobular, and belong to the duct system down to a diameter of about 



