280 



RESPIRATION 



is called the epiglottis. The whole of the larynx is lined by mucous mem- 

 brane, which, however, is extremely thin over the vocal cords. At its lower 

 extremity the larynx joins the trachea. 



Taste buds have been found in the epithelium of the posterior surface of 



the epiglottis, and in several other 

 situations in the laryngeal mucous 

 membrane. 



The Trachea and Bronchi. 

 The trachea extends from the 

 cricoid cartilage, which is on a 

 level with the fifth cervical vertebra, 

 to a point opposite the third dorsal 

 vertebra, where it divides into the 

 two bronchi, one for each lung, 

 figure 218. The trachea measures, 

 on an average, four or four and a 

 half inches, 12 to 14 cm., in length, 

 and from three-quarters of an inch 

 to an inch, 2 to 2.5 cm., in diameter, 

 and is essentially a tube of fibro- 

 elastic membrane within the layers 

 of which are enclosed a series of 

 cartilaginous rings, from sixteen to 

 twenty in number. These rings 

 extend only around the front and 

 sides of the trachea, about two- 

 thirds of its circumference, and 

 are deficient behind; the interval 

 between their posterior extremities 

 being bridged over by a continua- 

 tion of the fibrous membrane in 

 which they are enclosed, figure 

 219, h. 



Immediately within this tube and 



FIG. 219. Section of the Trachea, a, 

 Columnar ciliated epithelium; 6, and c, 

 proper structure of the mucous membrane, 

 containing elastic fibers cut across trans- 

 versely; d, submucous tissue containing 

 mucous glands, e, separated from the hya- 

 line cartilage, g, by a fine fibrous tissue,/, h, 

 external investment of fine fibrous tissue. 

 (S. K. Alcock.) 



at the back is a layer of unstriped 

 muscular fibers. This muscular layer extends transversely between the 

 ends of the cartilaginous rings to which it is attached, and also opposite the 

 intervals between them; its evident function being to diminish the caliber 

 of the trachea by approximating the ends of the cartilages. Outside there 

 are a few longitudinal bundles of muscular tissue, which, like the preceding, 

 are attached both to the fibrous and to the cartilaginous framework. 



The mucous membrane, figures 219 and 220, consists largely of adenoid 

 tissue, separated from the stratified columnar epithelium , which lines it, by a 



