THE RESPIRATORY SYSTEM 307 



The Larynx 



The larynx consists essentially of a group of cartilages united by 

 strong librous bands and lined with mucous membrane. 



The epithelium covering the true vocal cords, the laryngeal sur- 

 face of the epiglottis, and the anterior surface of the arytenoid carti- 

 lages is of the stratified squamous variety with underlying papillae. 

 With these exceptions the mucous membrane of the larynx is Hned 

 with stratified columnar ciHated epithelium similar to that of the re- 

 spiratory portion of the nares. Numerous goblet cells are usually 

 present, and the epithelium rests upon a broad basement membrane. 

 On the posterior surface of the epiglottis many taste buds (see page 

 593) are embedded in the epithelium. 



The stroma is especially rich in elastic fibres. The true vocal 

 cords consist almost wholly of longitudinal elastic fibres covered by 

 stratified squamous epithelium. Lymphoid cells are present in vary- 

 ing numbers. In some places they are so numerous that the tissue 

 assumes the character of diffuse lymphoid tissue. Distinct nodules 

 sometimes occur. 



Owing to the absence of a muscularis mucosae the stroma passes 

 over with no distinct line of demarcation into the submucosa. This 

 is a more loosely arranged, less cellular connective tissue, and con- 

 tains simple tubular glands lined with both serous and mucous cells. 



Externally the submucosa merges into a layer of more dense 

 fibrous tissue which connects it with the laryngeal cartilages and 

 with the surrounding structures. Immediately surrounding the 

 cartilages the connective tissue forms an extremely dense layer, the 

 perichondrium. 



Of the cartilages of the larynx, the epiglottis, the middle part of 

 the thyreoid, the apex and vocal process of the arytenoid, the carti- 

 lages of Santorini and of Wrisburg are of the yellow elastic variety. 

 The main body of the arytenoid, the rest of the thyreoid and the cri- 

 coid cartilages are hyaline. After the twentieth year, more or less 

 ossification is usually found in the cricoid and thyreoid cartilages. 



The Trachea 



The walls of the trachea consist of three layers — mucosa, submu- 

 cosa, and fibrosa (Fig. 206). 



The mucosa is continuous with that of the larynx, which it closely 

 resembles in structure. It consists of a stratified columnar ciliated 



