THE RESPIRATORY ORGANS 123 



THE RESPIRATORY ORGANS 



The larynx and trachea have the same general structure as the 

 larger bronchial tubes. The epithelium is stratified columnar and 

 ciliated, except that covering the surfaces of the epiglottis and that 

 of the upper part of the larynx, which is stratified squamous. 

 The cartilages are hyaline, except those of the epiglottis, part of 

 the arytenoids, and the cartilages of Wrisberg and Santorini, which 

 are yellow elastic cartilage. 



THE LUNGS 



At the root of each lung the large primary bronchus enters, 

 and divides into two branches, which also divide and branch 

 repeatedly until terminal or capillary bronchial tubes are formed, 

 which are one -fourth to one -eighth of a millimeter in diameter. 



A typical bronchial tube (Fig. 84) presents four coats, as 

 follows : 



1. Epithelial. 



2. Internal fibrous or mucosa. 



3. Muscular or muscularis mucosce. 



4. External fibrous or submiicosa. 



The lining epithelium is composed of cylindrical cells, provided 

 on their free extremities with delicate hair -like appendages the 

 cilia. Between the pointed, attached ends of the ciliated cells, 

 small, ovoid cells are wedged, and the whole rests upon a layer of 

 round cells. The epithelium pursues a wavy course, so that the 

 lumen of a tube appears stellate rather than circular in' transverse 

 section. This greatly increases the extent of surface. 



The internal fibrous coat or mucosa is composed of a small 

 amount of connective tissue, which, just beneath or outside the 

 epithelium, sustains collections of lymphoid (or adenoid) tissue. 

 In the pig, a considerable quantity of yellow elastic tissue is found 

 in the mucosa outside the lymphoid tissue, but the amount is 

 smaller in man. The fibers are, for the most part, disposed longi- 

 tudinally. Many nutrient vessels from the bronchial artery, cap- 

 illaries, venules, and lymph -spaces are also found in this coat. 



The muscular coat muscularis mucosae does not differ from 

 the same layer in other mucous membranes. Its thickness varies 



