440 HANDBOOK OF PHYSIOLOGY. 



The crico-arytenoidei postici muscles (Fig. 303) dilate the glottis, 

 and separate the vocal cords, the one from the other, by an action on 

 the arytenoid cartilage. By their contraction they tend "to pull together 

 the outer angles of the arytenoid cartilages in such a fashion as to rotate 

 the latter at their joint with the cricoid, and of course to throw asunder 

 their anterior angles to which the vocal cords are attached. 



These posterior crico-arytenoid muscles are opposed by the crico-ary- 

 tenoidei laterales, which, pulling in the opposite direction from the other 

 side of the axis of rotation, have of course exactly the opposite effect, 

 and close the glottis. 



The aperture of the glottis can be also contracted by the arytenoid 

 muscle (Fig. 305), which, in its contraction, pulls together the upper 

 parts of the arytenoid cartilages between which it extends. 



Nerve Supply. In the performance of the functions of the larynx 

 the sensory filaments of the superior laryngeal branch of the vagi sup- 



* wy epigloto 



CartiTV 

 Cart. San' 



. Arytefl 



pr. Crico-arytenoid. post. 

 C'ornu inferior 

 Lig. cerato-cric. 



Pars. post. inf. meml>rani 



Pars, caitflag 



FIG. 304. FIG. 305. 



FIG. 304. Cartilages of the larynx seen from the front. Ito4, thyroid cartilage; 1, vertical 

 ridge or pomum Adami; 2, right ala; 3, superior, and 4, inferior cornu of the right side; 5, 6, cricoid 

 cartilage; 5, inside of the posterior part; 6, anterior narrow part of the ring; 7, arytenoid cartilages. 



X ^ 



FIG. 305. The larynx as seen from behind. To show the intrinsic muscles posteriorly. (Stoerk ) 



ply that acute sensibility by which the glottis is guarded against the in- 

 gress of foreign bodies, or of irrespirable gases. The contact of these 

 stimulates the nerve filaments; and the impression conveyed to the 

 medulla oblougata, whether it produce sensation or not, is reflected to 

 the filaments of the recurrent or inferior laryngeal branch, and excites 

 contraction of the muscles that close the glottis. Both these branches 

 of pneumogastric co-operate also in the production and regulation of the 

 voice; the inferior laryngeal determining the contraction of the muscles 

 that vary the tension of the vocal cords, and the superior laryngeal con- 

 veying to the mind the sensation of the state of these muscles necessary 

 for their continuous guidance. And both the branches co-operate in 

 the actions of the larynx in the ordinary slight dilatation and contrac- 

 tion of the glottis in the acts of expiration and inspiration, and more 



