THE LARYNX. 



609 



Fig. 171.' 



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- 

 arytenpid, two posterior crico-arytenoid, two lateral crico- 

 arytenoid, and one arytenoid muscle. Their actions are as 

 follows : When the crico-thyroid muscles (10, fig. l/l) 

 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 arytenoid 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 mus- 

 cles (7, fig. 1/4), on the 

 other hand, have an 

 opposite action, pulling 

 the thyroid backwards, and 

 the arytenoid and upper and 

 back part of the cricoid car- 

 tilages forwards, and thus 

 relaxing the vocal cords. 



The crico-arytenoidei pos- 

 tici 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 reference to B' and c', (fig. 172). By their 



* Fig. 171. Lateral view of exterior of the larynx, after Mr. Willis. 

 8, Thyroid cartilage ; 9, Cricoid cartilage ; 10, Crico-thyroid muscle ; 

 n, Crico-thyroid ligament ; 12, first rings of trachea. 



R n 





