1080 THE VOICE. [BOOK m. 



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 ; and it has been urged that it may in 

 this action be assisted not antagonized by a part of the preceding 

 muscle. 



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

 in some respects stands apart from the rest, is the crico-thyroid 

 (M. crico-thyroideus anticus). This (Fig. 192 cr.th.) starts 

 from the front lateral surface of the cricoid, near its lower bor- 

 der, and passing obliquely backwards and upwards is inserted 

 into the lower edge and inner lateral surface of the thyroid. It 

 is sometimes 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 for- 

 wards over the front part of the cricoid, or, if the thyroid be 

 supposed to be the more fixed, to rotate the cricoid on its tran- 

 verse axis, pulling upwards the front part and tilting down- 

 wards the hind part on which the arytenoids sit ; the latter 

 is probably its real action. Upon either view, its contractions 

 increase the distance between the reentering angle of the 

 thyroid and the processus vocalis and so stretch the vocal cord ; 

 it is in fact the main tightener of the vocal cords. 



There are other small muscles in the larynx as well as mus- 

 cles connecting the larynx with surrounding parts ; but it is not 

 necessary for us to dwell on them here. Meanwhile it is ob- 

 vious from what we have said that narrowing or widening the 

 glottis, and slackening or tightening the vocal cords, are 

 brought about by the above muscles acting somewhat as 

 follows. 



670. Narrowing of the glottis ; adduction of the vocal cords. 

 The glottis is narrowed by the combined contraction of the three 

 muscles which we spoke of above as forming a sort of sphincter 

 for the larynx, namely, the transverse arytenoid, the oblique ary- 

 tenoid and the (external) thyro-arytenoid. These produce their 

 effect chiefly by bringing the two cartilages near to each other 

 in the middle line, and in this action the transverse arytenoid 

 muscle is the most potent. Hence this muscle may be regarded 

 as the most effective of the constrictors of the glottis. 



The glottis is also narrowed by the lateral crico-arytenoid, 

 but this produces its effect by rotation of the arytenoid carti- 

 lages ; it pulls the processus muscularis forwards and so throws 

 the processus vocalis inwards. 



Widening of the glottis ; abduction of the vocal cords. The 



