MUSCULAR MECHANISMS OF VOCAL CORDS. 1209 



THE MUSCULAR MECHANISMS WHICH AFFECT THE VOCAL CORDS. 



There is no doubt that the muscle which produces the greatest effect 

 in tightening the vocal cords, and thus raising the pitch of the voice, is 

 the thyro-cricoid. This muscle, acting from the thyroid cartilage as a 

 fixed point, rotates the cricoid in such a manner that its anterior part is 

 elevated towards the thyroid, and the posterior part, which bears the 

 arytenoid cartilages, is depressed and further removed from the angle 

 of the thyroid cartilage, thus stretching the thy ro -arytenoid ligaments, 

 the free edges of which, covered by mucous membrane, form the vocal 

 cords. There is, further, no doubt that the lateral crico -arytenoid 

 muscles must tend to rotate inwards the vocal processes of the arytenoid 

 cartilages, and thus to approximate or adduct the cords, and the posterior 

 crico-arytenoids must antagonise the lateral, rotate the vocal processes 

 outwards, and abduct the cords. The arytenoideus muscle, on the other 

 hand, brings the arytenoid cartilages bodily together, and at the same 

 time may somewhat tend to depress their vocal processes. The action 

 of the thyro -arytenoideus muscle is more complex, and will be imme- 

 diately considered. 



All the muscles except the thyro-cricoid, which receives its nerve 

 from the superior laryngeal, are innervated through the inferior laryngeal 

 branch of the vagus, the fibres being derived from the accessory roots. 



The inferior laryngeal contains, therefore, both the adductor and 

 abductor nerves. "When profoundly anaesthetised, stimulation causes 

 abduction of the cord, but if the anaesthesia is slight, then the same 

 stimulation will cause adduction. 1 



The tonic contraction of the abductors, produced by activity of a 

 centre in the medulla oblongata, is normally stronger than that of the 

 adductors, so that during rest the glottis is slightly open ; on section of 

 the inferior laryngeal nerve, the cords come together and the glottis 

 tends to close. The bulbar centre is under the dominance of a centre in 

 the Rolandic region of the cerebral cortex. 2 



With regard to the action of the thyro-arytenoid muscle, until 

 recently it has usually been held by English writers that the muscle, 

 when contracting as a whole, lowers pitch ; whereas German physio- 

 logists consider that the pitch of the voice is raised by it. 



Wyllie 3 found that, in the exsected larynx, when the cords were 

 approximated and the arytenoid cartilages pushed forward, the pitch of 

 the artificial voice, produced by blowing up through the trachea into the 

 larynx, was considerably raised. But it is not definitely stated by 

 Wyllie what is meant by pushing the arytenoid cartilages forward. If 

 these cartilages, as a whole, are pushed forward, and thus approximated 

 to the thyroid, the vocal cords cannot be tightened. Wyllie, hoover, 

 appears to mean that if the upper parts of the arytenoid cartilages be 

 pushed forward, a movement of rotation about a horizontal axis takes 

 place and the vocal processes are depressed, thus giving rise to increased 

 tension of the cords. It is difficult to see how the vocal processes 

 could be depressed during phonation, for the articular surfaces of the 

 cricoid cartilage are so placed that one would expect the vocal processes 

 of the arytenoids to rise during phonation. Neumann, 4 however, found 

 by experiment that the cord sank during phonation. 



1 Semon, Brit. Med. Journ., London, 1886, vol. ii. pp. 405 and 445. 



2 See article "Cerebral Cortex." 3 "Disorders of Speech," 1894, p. 482. 

 4 Monatschr. f. Ohrenh., Berlin, October 1894. 



