320 THE VOICE. [BOOK m. 



from the re-entering angle of the thyroid to the processus vocalis 

 and outer surface of the arytenoid forms a wedge-shaped mass, the 

 thin edge of which is covered by the actual vocal cord. It is called 

 the internal thyro-arytenoid (M. thyro-arytenoideus internus s. M. 

 vocalis) (Figs. 188, 189, m.th.ar.i. 184 A) and has by some authors 

 been subdivided into a median and lateral division. The general 

 direction of the muscle is horizontally backwards, but, as in the 

 external muscle, the constituent bundles run in various directions 

 and some are said to end or begin in the vocal cord itself. One 

 most important action of these two muscles is undoubtedly to 

 bring the arytenoids nearer to the thyroid and so to slacken the 

 vocal cords ; but they produce other effects, and their contractions, 

 especially those of the external muscle, help under circumstances 

 to bring the vocal cords together and so to narrow the glottis. 

 They also, as we shall see, produce changes in the form and thick- 

 ness of the cords. 



Of less importance than any of the above is a small muscle 

 which starting from the processus muscularis of one arytenoid 

 passes (Fig. 187-4, m.ar.o.) obliquely upwards towards the summit 

 of the other arytenoid, crossing its fellow obliquely at the back of 

 the transverse arytenoid muscle, which it thus partially covers ; 

 some of the fibres seem to end in the cartilage of Santorini but 

 most of them are continued to the thyroid, the ary-epiglottic 

 fold, and the base of the epiglottis. It is called the oblique aryte- 

 noid (M. arytenoideus obliquus) or it may be regarded as part of a 

 flat, irregular muscle, the thyro-ary-epiglottic muscle (Fig. 188 

 m. tk. ar. ep.). Its action is to approximate the two arytenoids and 

 so to help in closing the glottis. It, with the transverse arytenoid 

 and the external thyro-arytenoid 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. 187 m. cri. 

 ar. p.). This, starting from the lower part of the hind surface of 

 the cricoid near to the median line, passes obliquely upwards 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 and towards the 

 middle line, to throw the processus vocalis outwards and so to widen 

 the glottis ; it is in fact the special, we may perhaps say the only, 

 dilator of the glottis, or abductor of the cords. 



The above muscle meets its antagonist in the lateral crico- 

 arytenoid (M. crico-arytenoideus lateralis s. anterior) (Fig. 190 m. 

 cr. ar. I.), which taking origin from a large 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 



