RESPIRATION. 253 



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, bronchi, one to 

 each (right and left) lung. 



The Larynx is the upper part of the passage which leads exclusively 

 to the lung; it is formed by the thyroid, cricoid, and arytenoid cartilages 

 (fig. 200), and contains the vocal cords, by the vibration of which the 

 voice is chiefly produced. These vocal cords are ligamentous bands 

 attached to certain cartilages 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 chink between them, called the 

 r-ima 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, called the epiglottis (fig. 201, e). 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.* With the exception of the epiglottis and the so- 

 called cornicula laryngis, the cartilages of the larynx are of the hyalin 

 variety. 



The Epiglottis. The supporting cartilage of the epiglottis is com- 

 posed of yellow elastic cartilage, inclosed in a fibrous sheath (perichon- 

 drium), and covered on both sides with mucous membrane. The ante- 

 rior surface, which looks toward the back of the tongue, is covered with 

 mucous membrane, the basis of which is fibrous tissue, elevated toward 

 both surfaces in the form of rudimentary papillae, and covered with 

 several layers of squamous epithelium. In it ramify capillary blood- 

 vessels, and in its meshes are a large number of lymphatic channels* 

 Under the mucous membrane, in the less dense fibrous tissue of which 

 it is composed, is a number of tubular glands. The posterior or laryn- 

 geal surface of the epiglottis is covered by a mucous membrane, similar 

 in structure to that on the other surface, but its epithelial coat is thin- 

 ner, the number of strata of cells is less, and the papillae few and less, 

 distinct. The fibrous tissue which constitutes the mucous membrane is. 

 in great part of the adenoid variety, and is here and there collected into 

 distinct masses or follicles. The glands of the posterior surface are 

 smaller but more numerous than those of the other surface. In many 

 places the glands which are situated nearest to the perichondrium are 

 directly continuous through apertures in the cartilage with those on the 

 other side, and often the ducts of the glands from one side of the carti- 



* A detailed account of the structure and function of the Larynx will DA 

 found t in a later chapter. 



