868 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



part of the angle of the thyroid cartilage ; its fibres pass, for the most part, 

 backward and somewhat upward and outward to be inserted into the outer 

 edge of the arytenoid cartilage and its lateral processus muscularis (Figs. 295, 

 301). Some of its bundles of fibres, however, have different directions, and 

 a portion of them pass upward into the ventricular bands. The internal thyro- 

 arytenoid, wedge-shaped in transverse section, lies between the muscular divis- 

 ion just described and the vocal ligament, by which its thin median edge is 

 covered. The internal thyro-arytenoid arises from the anterior angle of the 

 thyroid cartilage and is inserted into the processus vocalis and the outer face of 

 the arytenoid cartilage. Certain fibre-bundles of this, as of the external 

 division of the muscle, pass in various directions, some of them being inserted 

 into the free border of the vocal cord. The action of the muscle is, on the 

 whole, to draw the arytenoids forward and thus relax the vocal cords ; but, by 

 its contraction, the cords may also be approximated and their thickness, and 

 probably their elasticity, extensively modified. 



Specific Actions of the Laryngeal Muscles. To sum up the various 

 effects of the muscular action on the larynx : A sphincter action of the larynx 

 is brought about by the combined contraction of all the muscles with the 

 exception of the crico-thyroids and the posterior crico-arytenoids; the vocal 



cords are adducted and the glottis nar- 

 rowed by the transverse and oblique ary- 

 tenoids, the external thyro-arytenoids, 

 an( j ^ ne lateral crico-arytenoids ; the 

 vocal cords are abducted and the glottis 

 yn.thy.ar. widened chiefly or wholly by the poste- 

 rior crico-arytenoids; the vocal cords 

 are made tense by contraction of the 

 crico-thyroids ; the vocal cords are slack- 

 ened by the combined action of the 



FIG. 301. Diagram to illustrate the thyro-aryte- sphincter group and especially by the 

 noid muscles; the figure represents a transverse 1 , .-, 



section of the larynx through the bases of the external thyro-arytenoids. 



arytenoid cartilages (redrawn from Foster) : Ary, j w [\\ easily be Seen that in the 



arytenoid cartilage ; p.m, processus muscularis ; , 



P.V, processus vocalis ; Th, thyroid cartilage ; c.v, larynx, as in the skeleton at large, the 



vocal cords; (E is placed in the oesophagus; e ffi c i ency o f any single muscle involves 

 m.thy.ar.i, internal thyro- arytenoid muscle; J 



m.thy.ar.e, external thyro-arytenoid muscle; the action of accessory muscles J thus, 

 m.thy.ar.ep, part of the thyro-ary-epiglottic mus- , /> ,1 .v 'A ,.^,,1,1 



cle, cut more or less transversely {m.ar.t, trans- Contraction of the CHCO-thyrOld could 



verse arytenoid muscle. have little effect in tightening the vocal 



cords were not the arytenoid cartilages 



fixed by contraction of the posterior crico-arytenoid and arytenoid muscles. 

 Nerve-supply of the Larynx. The larynx receives its nerve-supply from 

 the superior and the inferior or recurrent laryngeal nerves. The extremely 

 sensitive surface of the mucous membrane of the organ above the vocal cords 

 is supplied by sensory filaments of the superior laryngeal nerve. The superior 

 laryngeal also supplies motor fibres to the crico-thyroid muscle, whose action 

 as a tightener of the vocal cords is peculiar. All the other muscles of the 



