302 THE KESPIEATOEY SYSTEM 



THE TRACHEA 



The trachea proper extends from the lower border of the cricoid car- 

 tilage to the point where it bifurcates into the two primary bronchi, a 

 distance of about four and one-half inches. The wall of the trachea 

 somewhat resembles that of the larynx. It consists of three layers : 



1. The mucous membrane, or mucosa. 



2. The submucosa. 



3. The fibrocartilaginous coat, or adventitia. 



' The mucous membrane presents slight longitudinal folds, and is 

 lined by columnar ciliated epithelium, like that of the larynx, which 



IV 



FIG. 287. TRANSECTION OF THE WALL OF A CHILD'S TRACHEA. 



7, mucosa; II, submucosa; ///, cartilage; IV, outer fibrous coat; a, columnar 

 ciliated epithelium; b, tunica propria; c, layer of elastic fibers; d, mucous glands; 

 e, perichondrium. Hematein and eosin. Photo. X 90. 



rests upon a delicate basement membrane. The tunica propria includes 

 a thin inner layer of connective tissue which is richly supplied with 

 small blood-vessels and infiltrated by many lymphocytes, and an outer 



