THE LARYNX 245 



tion with the outside except by means of the respiratory passages. The air 

 enters these passages through the nostrils or through the mouth, thence it 

 passes through the larynx into the trachea or windpipe, which about the 

 middle of the chest divides into two tubes, the bronchi, one to each lung. 



The Larynx. The upper part of the passage which leads exclu- 

 sively to the lung is formed by the thyroid, cricoid, and arytenoid carti- 

 lages, figure 218, and contains the vocal cords, by the vibration of which the 

 voice is chiefly produced. These vocal cords are ligamentous bands covered 

 with mucous membrane and attached to certain cartilages which are capable 

 of movement by muscles. By their approximation the cords can entirely 

 close the entrance into the larynx; but under ordinary conditions the entrance 

 of the larynx is formed by a more or less triangular opening between them, 

 called the rima glottidis. Projecting at an acute angle between the base of 

 the tongue and the larynx, to which it is attached, is a leaf-shaped cartilage 

 with its larger extremity free. This is called the epiglottis. The whole of 

 the larynx is lined by mucous membrane, 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 mem- 

 brane. 



The Trachea and Bronchi. The trachea extends from the cricoid 

 cartilage, which is on a level with the fifth cervical vertebra, to a point oppo- 

 site 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 continuation of the fibrous membrane in which they are 

 enclosed, figure 219, h. 



Immediately within this tube and 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 

 homogeneous basement membrane. This is penetrated here and there by 

 channels which connect the adenoid tissue of the mucosa with the inter- 



