302 



THE KESPIKATORY 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 



III- 



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; 6, 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 



