144 THE RESPIRATORY SYSTEM. 



a mucosa. The mucous membrane on the anterior (or lingual, 

 or upper) surface of the epiglottis is a continuation of that of 

 the tongue and is very similar to it. 



The vocal cords are folds of the mucous membrane, with 

 the tunica propria reinforced and strengthened by additional 

 elastic fibres. 



The Trachea. 



The trachea is a rigid tube extending from the larynx to 

 the upper part of the chest, where it divides into the two 

 bronchi. Its walls are made up, from within outward, of 

 mucous membrane, submucous tissue, cartilage and muscle, 

 and fibrous tissue. 



The mucous membrane of the trachea is lined with stratified 

 ciliated epithelium-cells (Fig. 14), among which are occasional 

 goblet-cells. The tunica propria underlying the epithelium is 

 a firm supporting connective-tissue layer ; its most superficial 

 layer forms a basement-membrane for the epithelium ; below 

 this the tunic is infiltrated with numerous lymphoid cells, 

 and its deepest portion contains numerous longitudinal elastic 

 fibres. 



Outside the tunica propria is the submucosa, a layer of 

 areolar and elastic tissue ; it contains the alveoli of numerous 

 small racemose mucous glands, occasional lymphoid tissue, and 

 the larger blood and lymphatic vessels. 



Next outside the submucosa is a series of incomplete rings 

 or horseshoe-shaped bodies of hyaline cartilage, defective 

 posteriorly, which makes up the rigid incollapsible framework 

 of the trachea. The connective tissue on the surfaces of the 

 cartilage is condensed to form a perichondrium. 



The hiatus at the posterior part of the cartilages is filled up 

 partially by fibrous tissue, but chiefly by the trachealis muscle, 

 a bridge of involuntary muscle which connects the ends and 

 posterior surfaces of the cartilages ; the larger portion of this 

 muscle is arranged transversely, but externally there are a few 

 longitudinal fibres. 



There are some mucous glands located posterior or external 

 to the trachealis muscle, whose ducts pierce the muscle and 

 find their way to the lumen of the trachea. 



