THE RESPIRATORV SYSTEM 



311 



single layer of ciliated cells. These are at lirst high, but become 

 gradually lower as the bronchi become smaller, until in the terminal 

 branches the epithelium is simple cuboidal and non-ciliated. Among 

 the cih'ated cells are varying numbers of mucous or goblet cells. 



(2) The stroma decreases in thickness as the bronchi become 

 smaller. It consists of loosely arranged white and elastic fibres. 

 This layer with the epithehum is folded longitudinally (Fig. 21 i). 

 There is considerable diffuse lymphatic tissue, and in some places 

 small nodules occur, over which there may be lymphoid infiltration 



/ 



Fig. 208. — Transverse Section through two Medium-size Bronchi of the Human 

 Lung. X15. (Technic 2, p. 323) In the fibrous coat are seen the bronchial arteries 

 and veins, a, Epithehum; b, stroma; c, muscularis mucosse; d, Umg tissue; e, fibrous 

 coat; /, plates of cartilage. 



of the epithelium (see Tonsil, page 183). Near the root of the lung 

 many small lymph nodules are found, which show different degrees of 

 pigmentation. 



(3) With decrease in thickness of the epithelium and of the 

 stroma, the thickness of the mucosa is maintained by the appearance 

 of a layer of smooth muscle. In the larger bronchi this is a contin- 

 uous layer of circularly disposed smooth muscle, and lies just external 

 to the stroma, forming a muscularis mucosae (Fig. 209) . It reaches its 

 greatest thickness relative to the size of the bronchus in the bronchi 

 of medium size. As the bronchi become smaller it becomes thinner, 

 then discontinuous, and in the smallest bronchi consists of only a 

 few scattered muscle cells. These continue into the walls of the 

 alveolar ducts, but are absent beyond this point. 



(4) The submucosa decreases in thickness with decrease in the 



