CHAP. VIL] SPECIAL MUSCULAR MECHANISMS. 



1459 



arytenoid in front of and below the insertion of the posterior 

 crico-arytenoid. Its main action is to wheel the outer corner of 

 the arytenoid forwards and inwards and thus, by converging the 

 processus vocales, to adduct the cords and to narrow the glottis ; 

 but it may, under circumstances, have other effects. 



The last muscle to which we need call attention, and which in 

 some respects stands apart from the rest, is the crico-ihyroid 

 (M. crico-thyroideus anticus). This (Fig. 191 cr. th.) starts 



p.v 



m.cr ar.L- 



m.cr. th.r 



m.cr.th.o 



FIG. 190. FIG. 191. 



FIG. 190. THE LATERAL CRICO-ARYTENOID MUSCLE. 



Ary. Arytenoid. p.v. processus vocalis. p.m. processus muscularis. Cri. Cri- 

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

 lateral crico-arytenoid muscle. 



FIG. 191. THE CRICO-THYROID 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-thyroid 

 membrane. 



from the front lateral surface of the cricoid, near its lower border, 

 and passing obliquely backwards and upwards is inserted into the 

 lower edge and inner lateral surface of the thyroid. It is some- 

 times subdivided into a front part (cr. th. r.) the fibres of which 

 run more directly upwards (M. cr. thy. rectus) and a lateral part 

 (cr. th. o.) the fibres of which run in a more oblique direction 

 (M. cr. thy. obliquus). The action of the muscle is a somewhat 

 complicated one, but the effect of its contractions as a whole is, 

 if the thyroid be regarded as the more moveable of the two 

 cartilages, to pull the thyroid downwards and forwards over the 

 front part of the cricoid, or, if the thyroid be supposed to be the 

 more fixed, to rotate the cricoid on its transverse axis, pulling 

 upwards the front part and tilting downwards the hind part on 

 which the arvtenoids sit; the latter is probably its real action. 

 Upon either view, its contractions increase the distance between 



F. 



93 



