322 THE VOICEo [BOOK m. 



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 muscles 

 connecting the larynx with surrounding parts ; but it is not 

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

 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. 



907. 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 arytenoid 

 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 cartilages ; it 

 pulls the processus muscularis forwards and so throws the proces- 

 sus vocalis inwards. 



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

 chief if not the only agent for the widening of the glottis is the 

 posterior crico-arytenoid. This, pulling the outer edge of the 

 arytenoid backwards, throws the processus vocales outwards, and 

 so abducts the vocal cords. It has been argued that the transverse 

 arytenoid acting alone or in concert with the above, or at least in 

 the absence of any contraction of the other members of the 

 sphincter group, would also wheel the outer edge of the arytenoid 

 in the same way and so also abduct the vocal cords ; but the 

 evidence seems to be against this view. 



Tightening of the vocal cords. This is especially effected by 

 one muscle on each side, namely by the crico-thyroid which, by 

 bringing the thyroid and the front -part of the cricoid nearer to 

 each other, increases the distance between the thyroid and the 

 arytenoids when the latter are fixed. Supposing the transverse 

 arytenoid and posterior crico-arytenoid to fix the arytenoids, the 

 direct effect of the contraction of the crico-thyroid is to tighten 

 the vocal cords. There is besides a special action of the internal 

 thyro-arytenoid by which this muscle becomes, in contrast to the 

 external thyro-arytenoid, a tightener of the cord ; of this action 

 we shall speak later on. 



Slackening of the vocal cords. This is effected by the whole 

 sphincter group just mentioned, but more especially by the ex- 

 ternal thyro-arytenoid and to a certain extent by the internal 

 thyro-arytenoid ; these acting alone, produce an effect the reverse 

 of that of the crico-thyroid, bringing the arytenoid cartilages nearer 



