VOICE AND SPEECH 



521 



1 



(2) Sliding movements of the whole arytenoid cartilage either outwards or inwards, 

 so that their inner margins may be drawn apart or approximated. 



The larynx is covered internally by a mucous membrane continuous with that of 

 the trachea. It is lined with ciliated epithelium, except over the vocal cords, where 

 the epithelium is stratified. The two vocal cords, or thyro-arytenoid ligaments, con- 

 sist of elastic fibres which run from the middle of the inner angle of the thyroid carti- 

 lage to be inserted into the anterior angle of the arytenoid cartilages. Their length 

 in man is about 15 mm., in woman about 11 mm. 

 The cleft between them is known as the glottis, or 

 rima glottidis. 



Two ridges of mucous membrane above and 

 parallel to the vocal cords are the false vocal 

 cords (Fig. 253). Between the true and the false 

 vocal cords on each side is a recess known as the 

 ventricle of Morgagni. This ventricle permits the 

 free vibration of the vocal cords. The false cords 

 take no part in phonation, but help to keep the 

 true cords moistened by the secretion of the 

 numerous mucous glands with which they are 

 provided. The position and tension of the vocal 

 cords are determined by the action of the various 

 intrinsic muscles of the larynx. The part taken 

 by the various muscles in each movement cannot 

 be directly ascertained. We can in most cases 

 only study the direction of the fibres, and judge, 

 from this direction and consequent isolated action 

 of the muscles, the part taken by any given muscle 

 in the production of voice. The chief muscles 

 (Fig. 254) are as follows : 



(1) The crico-thyroid muscle is a short triangular 

 muscle attached below to the cricoid cartilage and 

 above to the inferior border of the thyroid carti- 

 lage ; the fibres pass from below upwards and 

 backwards. When this muscle contracts, the 

 cricoid cartilage is drawn up under the anterior 

 part of the thyroid cartilage so that its broad 

 expansion behind with the arytenoid cartilages, 

 is drawn downwards and backwards, thus putting 

 the vocal cords on the stretch. This muscle is 

 probably the most important in determining the 

 tension of the vocal cord. 



(2) The posterior crico-arytenoid muscle arises 

 from a broad depression on the corresponding 



half of the posterior surface of the cricoid cartilage. It passes upwards and outwards, 

 its fibres converging, to be inserted into the outer angle of the arytenoid cartilage. 

 These muscles rotate the outer angle of the arytenoid cartilages backwards and 

 inwards. They thus cause a movement outwards of the anterior angles, so that 

 the glottis is widened. During every act of inspiration there is a widening of the 

 glottis, which is probably effected by contraction of these muscles. If they are paralysed 

 the vocal cords are approximated and tend to come together during inspiration, so that 

 dyspnoea may be produced. 



(3) The lateral crico-arytenoid muscle arises from the upper border of the cricoid 

 cartilage and passes backwards to be inserted into the muscular process of the arytenoid 

 cartilage. These muscles when they contract pull the muscular process of the arytenoid 

 cartilage forwards and downwards, thus approximating the vocal cords at their posterior 

 ends and antagonising the action of the posterior crico-arytenoid muscles. 



(4) The arytenoid muscles consist of transverse fibres, some of which decussate, 



17* 



FIG. 253. Anterior half of the larynx, 

 seen from behind. The section on 

 the right side is somewhat in front 

 of the left side. 



e, epiglottis ; e', cushion of epi- 

 glottis ; t, thyroid cartilage ; s, s', 

 ventricle of larynx ; h, great cornu 

 of hyoid bone ; t a, thyro-arytenoid 

 muscle; vl, vocal cords. Above the 

 ventricles are the false vocal cords. 

 r, first ring of trachea. 



(A. THOMSON.) 



