178 JAY ARTHUR MYERS 



in the region of the tracheal bifurcation, are quite near the sur- 

 face, since they he just beneath the epithehum. Where these 

 lymph nodules occur the epithelial cells are non-ciliated. 



A distinct submucosa is present throughout practically the en- 

 tire extent of the mucous membrane of the trachea and the 

 bronchi, and connects this mucosa to the interannu'ar membranes 

 and the periosteum, or the perichondrium of the tracheal rings, 

 •the intermediate syringeal cartilages, and the bronchial half-rings. 

 There is no sharp line of demarcation between the submucosa 

 and the tunica propria, but the former is easily distinguished 

 from the latter in that it is less dense and contains fewer lymph 

 cells. Over the tympanic membranes the tunica propria s thin 

 and sparse and a submucosa is indistinguishable. 



At the level of the caudal border of the third tracheal ring, 

 the epithelial cells loose their cilia and immediately become col- 

 umnar in shape. There is a gradual transition from stratified 

 ciliated columnar epithelium to stratified squamous. The lat- 

 ter is made up of several layers at the lower border of the tym- 

 panum, but becomes thinner and thinner, as far as a point slightly 

 caudad to the fourth intermediate syringeal cartilages, where it 

 consists of only a single layer of flat cells, covering the internal, 

 or medial, surfaces of the external tympanic membranes. The 

 lateral surfaces of the internal tympanic membranes, or those 

 surfaces of these membranes which face the lumen of each pri- 

 mary bronchus, are covered by a similar simple epithelium. 

 Below the limits of the external tympanic membranes the epi- 

 thelium gradually becomes thicker, and about midway between 

 the first and second bronchial half-rings it again assumes the 

 characteristics of a ciliated columnar epithelium . . 



The disposition of the squamous epithelium suggests that the 

 portions of the sja-ingeal walls covered by it are subject to con- 

 siderable movement. 



As stated above, the vibrating membranes are covered by a 

 thin epithe'ium composed o a single layer of flattened cells; 

 beneath this is the tunica propria which is thin and ess distinct 

 than that found in other parts of the respiratory tract. Beneath 

 the tunica propria is a thin, dense layer of tissue which comprises 



