CHAPTER XIII. 

 THE RESPIRATORY ORGANS. 



THE respiratory tract consists of the larynx, trachea, bronchi, 

 and lungs. 



1. The Larynx. The interior of the larynx is lined with ciliated 

 columnar epithelium, which extends over the false vocal cords and 

 about half-way up the epiglottis above, and is continuous below 

 with a similar lining throughout the trachea and bronchi. This 

 lining is interrupted over the true vocal cords by a covering of 

 stratified epithelium, and at its upper limits passes into the stratified 

 epithelium lining the buccal cavity and pharynx and covering the 

 tongue. Opening upon this epithelial surface, except upon the true 

 vocal cords and in the smallest bronchi, are mucous glands, varying 

 in number in different situations. Some of the columnar cells upon 

 the surface are also mucigenous, discharging their secretion upon 

 the free surface of the mucous membrane. 



The thyroid, cricoid, and most of the arytenoid cartilages are 

 composed of the hyaline variety of that tissue : the epiglottis, 

 cornicula laryngis, and the apices of the arytenoids, of elastic car- 

 tilage. 



Beneath the epithelium lining the laryngeal ventricle is a con- 

 siderable layer of lymphadenoid tissue. In other situations the 

 epithelium rests upon fibrous tissue. 



2. The Trachea. The tracheal wall may be divided into four 

 coats : a, the mucous membrane ; 6, the subrnncous coat ; c, the 

 cartilage ; d, the fibrous coat (Fig. 145). 



a. The mucous membrane is covered with ciliated columnar epi- 

 thelium resting upon a nearly homogeneous basement-membrane, 

 beneath which is a layer of fibrous tissue. This may be divided 

 into two portions : an outer one, next to the basement-membrane, 

 which is areolar in character, with a large admixture of elastic 

 fibres and lymphadenoid tissue, and an abundant supply of blood- 

 vessels ; and an inner one, less highly vascularized, and composed 

 chiefly of elastic fibres running a longitudinal course. 



168 



