Chap, vii.] SPECIAL MUSCULAR MECHANISMS. 1079 



imate the two arytenoids and so to help in closing the glottis. 

 It, with the transverse arytenoid and the external thyroaryte- 

 noid muscles, may be looked upon as forming together a sort of 

 sphincter of the larynx ; their combined contractions certainly 

 tend to close the glottis. 



A relatively large and very important muscle is the posterior 

 crico-arytenoid (M. crico-arytenoideus posticus) (Fig. 188 

 m.cri.ar.p.~). This, starting from the lower part of the hind sur- 

 face of the cricoid near to the median line, passes obliquely up- 

 wards to be inserted into the outer edge of the arytenoid just 

 below the insertion of the transverse arytenoid muscle, at the 

 upper part of the processus muscularis. Its chief action is by 

 wheeling the outer corners of the arytenoids backwards to throw 

 the processus vocalis outwards and so to widen the glottis ; it is 

 in this way a special, we may perhaps say the only, dilator of the 

 glottis, or abductor of the cords ; but it is maintained that it 

 has other actions. 



The above muscle acting as a dilator meets its antagonist in 

 the lateral crico-arytenoid (M. crico-arytenoideus lateralis s. ante- 

 rior) (Fig. 191 m.cr.ar.l.% which taking origin from a large 



m.cr ar.1 



m.cr.th.o 



Fig. 191. Fig. 192. 



Fig. 191. The Lateral Crico-arytenoid Muscle. 



Ary. arytenoid, p. v. processus vocalis. p. m. processus muscularis. Cri. 

 cricoid. 1. surface for articulation of lower cornu of thyroid, m.cr.ar. I. the 

 lateral crico-arytenoid muscle. 



Fig. 192. The Crico-thtroid Muscle. 



Th. thyroid; c.i. its inferior cornu. Cri. cricoid; m.cr.th.r. the straight 

 part, m.cr.th.o. the oblique part of the crico-thyroid muscle, m.cr.th. crico-thy- 

 roid membrane. 



portion of the upper border of the cricoid cartilage in its lateral 

 parts in front of the thyro-cricoid articulation, passes upwards 

 and backwards to be inserted into the processus muscularis and 

 outer side of the arytenoid in front of and below the insertion 



