THE LARYNX. 



479 



FIG. 171. 



tilages rest on the top of the back portion of the cricoid car- 

 tilage (a, Fig. 169), and are connected with it by capsular and 

 other ligaments, all movements of the cricoid cartilage must 

 move the arytenoid cartilages, and also produce an effect on 

 the vocal cords. 



The so-called intrinsic muscles of the larynx, or those which, 

 in their action, have a direct action on the vocal cords, are 

 nine in number four pairs, and a single muscle; namely, two 

 crico-thyroid muscles, two thyro-arytenoid, two posterior crico- 

 arytenoid, two lateral crico-arytenoid, and one arytenoid muscle. 

 Their actions are as follows : When the crico-thyroid muscles 

 (10, Fig. 171) contract, they 

 rotate the cricoid on the thyroid 

 cartilage in such a manner that 

 the upper and back part of the 

 former, and of necessity the aryt- 

 enoid cartilages on the top of 

 it, are tipped backwards, while 

 the thyroid is inclined forward ; 

 and thus, of course, the vocal 

 cords being attached in front to 

 one, and behind to the other, 

 are "put on the stretch." 



The thyro-arytenoid muscles 

 (7, Fig. 174), on the other hand, 

 have an opposite action pull- 

 ing the thyroid backwards, and 

 the arytenoid and upper and 

 back part of the cricoid carti- 

 lages forwards, and thus relaxing 

 the vocal cords. 



The crico-arytenoidei postici 

 muscles (Fig. 173, b) dilate the 

 glottis, and separate the vocal 

 cords, the one from the other, 

 by an action on the arytenoid 

 cartilage, which will be plain on C hea. 

 reference to B' and c', Fig. 172. 

 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 at- 

 tached. 



These posterior crico-arytenoid muscles are opposed by the 

 crico-arytenoidei laterales, which, pulling in the opposite direc- 

 tion from the other side of the axis of rotation, have of course 



Lateral view of exterior of the 

 larynx, after Mr. Willis. 8, thyroid 

 cartilage ; 9, cricoid cartilage ; 10, 

 crico-thyroid muscle; 11, crico-thy- 

 roid ligament; 12, first rings of tra- 



