THE TRACHEA 



303 



layer of elastic tissue most of whose fibers 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 fibers are more numerous in the trachea of the lower 

 mammals than in that of man. A muscularis mucosa, a characteristic 

 structure of the mucosa of the digestive tube, 

 is lacking in the trachea. The elastic mem- 

 brane occupies the position held by the mus- 

 cularis mucosa in other organs. 



The submucosa consists of loose areolar 

 tissue which contains many small tubulo-acinar 

 mucous glands. The ducts of these glands 

 penetrate the mucosa and open upon the free 

 surface of the trachea. They supply an abun- 

 dant mucous secretion. This coat also contains 

 the larger blood-vessels and nerves which are 

 destined for the supply of the mucosa. 



The fibrocartilaginous coat is formed by 

 the C-shaped 'ring cartilages' of the trachea 

 (from sixteen to twenty in number) which are 

 firmly united to one another by ligamentous 

 membranes of fibrous tissue continuous with 

 the perichondrium of adjacent cartilage plates. 

 The cartilages are of the hyaline variety and 

 are subject to more or less ossification as age 

 advances. They rarely overlap each other, so 

 that but a single plate of cartilage forms the 

 wall at any given point. Their borders are 

 irregular, and horizontal sections near the 



upper or lower margin of the cartilage frequently pass through several 

 projections, which, unless properly interpreted, would lead one to infer 

 that the cartilaginous plate was interrupted. . 



The interval between the ends of the C-shaped cartilage plates is 

 occupied by a membrane of smooth muscle whose transverse fibers unite 

 the adjacent ends of the cartilages. The muscle fibers are inserted into 

 the perichondrium of the cartilages. Many of the fibers are obliquely, 

 and a few of the outermost are longitudinally, disposed. This muscular 

 portion of the tracheal wall forms the so-called track ealis muscle. The 

 mucous membrane and submucosa of this portion of the trachea are un- 

 usually thick and their mucous glands are exceptionally large. The 



FIG. 288. MUCUS-SE- 

 CRETING, TUBULO - AL- 

 VEOLAR GLAND OF THE 

 HUMAN TRACHEAL MU- 

 COSA. 



The terminal dark areas 

 are demilunes. Recon- 

 struction. X 200. (After 

 Maziarski.) 



