CHAP, vii.] SPECIAL MUSCULAE MECHANISMS. 1081 



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 out- 

 wards, 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 mem- 

 bers 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 con- 

 trast 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 re- 

 verse of that of the crico-thyroid, bringing the arytenoid 

 cartilages nearer to the thyroid cartilage, and so shortening 

 the distance between the processus vocales and that body. 



These several acts, however, the widening or narrowing of 

 the glottis, the tightening or slackening of the vocal cords, are 

 only the gross acts, so to speak, of the movements of the larynx. 

 When a voice of any kind has to be uttered the cords must 

 be approximated and to a certain extent tightened; and for 

 the carrying out of even these gross acts not one muscle only 

 but more than one, and often several are brought into play ; the 

 movements which give rise to any kind of voice are combined 

 and coordinated movements. But, as we shall see presently, 

 when this or that particular kind of voice is being uttered or 

 when changes in the voice are being effected, the above words, 

 widening, tightening and the like, very imperfectly describe 

 what is taking place in the larynx ; changes of a very complex 

 nature are brought about, and for these the greatest nicety of 

 combination is necessary. 



671, We may now turn to the nervous mechanisms of the 

 larynx. Fibres of the superior laryngeal branch of the vagus 

 nerve are distributed to the mucous membrane of the larynx, 

 and serve as the afferent channels by which impulses from the 

 exquisitely sensitive surface pass upwards to the central ner- 

 vous system. 



