234: 



THE RESPIRATORY SYSTEM 



THE TRACHEA. The wall of the trachea somewhat resem- 

 bles that of the larynx. It consists of three layers : 



1. The mucous membrane. 



2. The submucosa. 



3. The fibro-cartilaginous coat. 



The mucous membrane presents slight longitudinal folds, and is 

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

 which 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 



II 



IV ^ 



FIG. 198. TRANSECTION OF THE WALL OF A CHILD'S TRACHEA. 



/, mucosa ; 77, submucosa ; 777, cartilage ; 7F, outer fibrous coat ; a. columnar ciliated 

 epithelium ; 6, tunica propria ; c, layer of elastic fibres ; a5, mucous glands \ e, perichon- 

 drium. Hematein and eosin. Photo, x 90. 



leucocytes, and an outer layer of elastic tissue 'most of whose fibres 

 are longitudinally disposed. The elastic layer begins in the region 

 of the vocal cords in the larynx and is continuous below with the 

 similar layer of the bronchial mucous membrane. Elastic fibres 

 are more numerous in the trachea of the lower mammals than in 

 that of man. 



