486 MANUAL OF PHYSIOLOGY. 



MECHANISM OF VOCALIZATION. 



Taking the thyroid cartilage as the fixed base, the cricoid and 

 arytenoid cartilages updergo movements which bring about two 

 distinct sets of changes in the glottis and its elastic edges, namely, 

 (1) widening and narrowing the opening; (2) stretching and re- 

 laxing of the vocal cords. During ordinary respiration the glottis 

 remains about half open, the muscles being in a state of relaxa- 

 tion (B') During forced inspiration the glottis is widely dilated 

 by muscular action (C'). If an irritating gas be inspired, the 

 glottis is tightly closed by a spasmodic action of certain muscles, 

 so that the true vocal cords act as a kind of valve. 



During vocalization the glottis is formed into a narrow chink 

 with parallel sides (A'), while the cords are made more or less 

 tense, according to the pitch of the note to be produced ; both 

 these changes being brought about by muscular action. 



The opening of the chink of the glottis is accomplished chiefly 

 by a muscle called the posterior crico-arytenoid, which passes from 

 the posterior surface of the cricoid cartilage to the outer and poste- 

 rior angle of the arytenoids. By pulling the latter point down- 

 wards and backwards it separates the arytenoid cartilages, 

 particularly at their anterior extremity, where the cords are at- 

 tached, In this action they are aided by a small muscle con- 

 necting the posterior surfaces of the arytenoid, namely, the 

 posterior arytenoid, which tends, when the two arytenoid cartilages 

 are held apart, to rotate them so that the vocal processes are 

 separated. 



The narrowing of the glottis is executed by the lateral crico- 

 arytenoids which run upwards and backwards from the autero- 

 lateral aspect of the cricoid to the muscular processes of the ary- 

 tenoid cartilages. They pull the muscular processes forwards, 

 and thus rotate the arytenoid cartilages so as to approximate the 

 vocal processes to one another, while any tendency towards pull- 

 ing apart the bodies of the cartilages, owing to the downward 

 direction of the muscle, is overcome by the posterior arytenoid 

 muscle and those muscular bauds which pass from the posterior 

 surface of the arytenoid cartilages to the epiglottis and the upper 



