ARRANGEMENT OF THE LARYNX. 



603 



The tension of the vocal bands is effected by the action of muscles in 

 separating their two points of attachment from each other. To this 

 end the thyroid cartilage is drawn forward and downward chiefly by the 

 crico-thyroid muscles, the angle of this cartilage being at the same time 

 somewhat enlarged. One can readily convince himself of this move- 

 ment by feeling his own larynx during the emission of high tones. 

 The same muscles also approximate the anterior arch of the cricoid 

 cartilage to the inferior border of the thyroid cartilage ; and as a result 

 the posterior plate of the cricoid cartilage undergoes a backward in- 

 clination. At the same time the posterior crico-arytenoid muscles 

 must draw both arytenoid cartilages somewhat backward, and hold 

 them in that position. The tense vocal bands become longer and nar- 

 rower. 



FIG. 208. Diagrammatic Horizontal Section through 

 the Larynx: 7, /, Position of the arytenoid carti- 

 lages during respiration, in horizontal section; 

 from their anterior angles run the convergent vocal 

 bands to the internal angle of the thyroid cartilage. 

 The arrows indicate the direction of traction of the 

 posterior crico-arytenoid muscles. //, //, Posi- 

 tion of the arytenoid cartilages as a result of the 

 action of these muscles. 



FIG. 209. Diagrammatic Horizontal Section through 

 the Larynx, to Illustrate the Action of the Aryte- 

 noid Muscle : /, /, Position of the arytenoid carti- 

 lages during quiet respiration. The arrows indi- 

 cate the direction of traction of the muscle. II, II, 

 Positions of the arytenoid cartilages produced by 

 the action of this muscle. 



The tension of the vocal bands is aided by the genio-hyoid and hyo-thyroid 

 muscles, which together draw the hyoid bone, and thus indirectly the thyroid 

 cartilage, upward and forward in the direction of the chin. 



According to Harless, Schech, Kiesselbach, Hooper, and others, the crico-thyroid 

 muscle effects elevation of the arch of the cricoid cartilage toward the thyroid 

 cartilage. In this way the plate of the cricoid cartilage is directed backward and 

 downward, thus causing increased tension of the vocal bands. 



Pathological. Paralysis of the crico-thyroid muscles renders the voice harsh 

 and deeper, on account of insufficient tension of the vocal bands. 



The tension thus induced is of itself by no means sufficient for pho- 

 nation, for on the one hand the triangular aperture of the glottis between 

 the arytenoid cartilages that would result from the isolated action of 

 the internal thyro-arytenoid muscles must be closed. This is brought 

 about by the transverse and oblique posterior arytenoid muscles. Then 

 the vocal bands themselves, which, with the action of the crico-thyroid 

 and posterior crico-arytenoid muscles, retain their concave border, so 



