Ill 



PHONATION AND ARTICULATION 



139 



they cannot be drawn forward. This is effected by the combined 

 action of the dilatators and constrictors of the glottis, viz. the 

 posterior crico-arytenoids (dilatators), the transverse and oblique 

 arytenoids, the external thyro-arytenoids, and the lateral crico- 

 arytenoids (constrictors). If the posterior crico-arytenoids alone 

 contracted with the crico-thyroids, the vocal cords would be 

 stretched and abducted and the glottis dilated. But it is 

 essential for the formation of sounds that the cords shall be not 

 only tense, but also approximated to each other, so that they can be 

 thrown into vibration by the expiratory air-current. These two 

 conditions are realised when the constrictors of the glottis are 

 thrown into action simultaneously with the dilatators. 



According to C. Meyer and Griitzner, the genio-hyoid and 

 thyro-hyoid muscles con- 

 tribute to the tension of 

 the vocal cords, as they 

 raise the thyroid upwards 

 and forwards in the direc- 

 tion of the chin, and sup- 

 plement the action of the 

 crico- thyroid muscles by 

 which the rotation of the 

 crico- thyroid articulations 

 round the transverse hori- 

 zontal axis is effected. 



The relaxation of the 

 vocal cords is due to 

 simple elastic reaction 

 when the extensor muscles 

 cease to act. Active re- 

 laxation of the cords can, 



however, be produced by the internal thyro-arytenoids, which 

 are perhaps the most important muscles for phonation. They 

 are triangular muscles, which extend with the vocal cords from 

 the inner angle of the thyroid to the vocal processes of the 

 arytenoids, but some of their bundles are inserted in the elastic 

 substance of the cords. When these muscles contract they pro- 

 duce an opposite effect to the crico-thyroids, and bring the vocal 

 processes of the arytenoid nearer to the thyroid, which relaxes 

 the cords. But it is conceivable that contraction of the isolated 

 bundles inserted into the elastic tissue of the cords may produce 

 tension of some parts and relaxation of others. 



It is very probable that the true function of the internal 

 thyro-arytenoids in phonation is to regulate the tension and 

 thickness of the vibrating portion of the vocal cords, by which 

 a rapid succession of tones of different pitch is made possible. 



The internal thyro-arytenoids almost always co-operate in 



FIG. 95. Transverse section of larynx through bone of 

 arytenoid cartilages. (Henle.) t, thyroid ; Pi-, pro- 

 cessus vocalis of arytenoid ; Sp, sinus pyriformis ; 

 Th, section through thyro - hyoid muscle ; A, ary- 

 tenoid muscle; Toe, Tai, thyro - arytenoid muscles, 

 internal and external ; Taep, thyro-ary-epiglottidean 

 muscle ; *, anterior cord of glottis. 



