258 HUMAN PHYSIOLOGY. 



VOICE AND SPEECH. 



The larynx is the organ of voice. Speech is a modification of 

 voice, and is produced by the teeth and the muscles of the lips and 

 tongue, coordinated in their action by stimuli derived from the 

 cerebrum. 



The structures entering into the formation of the larynx are 

 mainly the thyroid, cricoid, and arytenoid cartilages ; they are so 

 situated and united by means of ligaments and muscles as to form a 

 firm cartilaginous box. The larynx is covered externally by fibrous 

 tissue, and lined internally with mucous membrane. 



The vocal cords are four ligamentous bands, running anteropos- 

 teriorly across the upper portion of the larynx, and are divided 

 into the two superior or false vocal cords, and the two inferior or true 

 vocal cords ; they are attached anteriorly to the receding angle of 

 the thyroid cartilages, and posteriorly to the anterior part of the 

 base of the arytenoid cartilages. The space between the true vocal 

 cords is the rima glottidis. 



The muscles which have a direct action upon the movements of 

 the vocal cords are nine in number, and take their names from their 

 points of origin and insertion viz., the two crico-thyroid, two 

 thyro-arytenoid, two posterior crico-arytenoid, two lateral crico-ary- 

 tenoid, and one arytenoid muscles. 



The crico-thyroid muscles, by their contraction, render the vocal 

 cords more tense by drawing down the anterior portion of the 

 thyroid cartilage and approximating it to the cricoid, and at the 

 same time tilting the posterior portion of the cricoid and arytenoid 

 cartilages backward. 



The thyro-arytenoid, by their contraction, relax the vocal cords 

 by drawing the arytenoid cartilage forward and the thyroid back- 

 ward. 



The posterior crico-arytenoid muscles, by their contraction, rotate 

 the arytenoid cartilages outward and thus separate the vocal cords 

 and enlarge the aperture of the glottis. They principally aid the 

 respiratory movements during inspiration. 



The lateral crico-arytenoid muscles are antagonistic to the former, 

 and by their contraction rotate the arytenoid cartilages so as to ap- 

 proximate the vocal cords and constrict the glottis, 



