174 HUMAN PHYSIOLOGY. 



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-arytenoid, 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 pos- 

 terior portion of the cricoid and arytenoid cartilages backward. 



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

 ing the arytenoid cartilage forward and the thyroid backward. 



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 approximate the 

 vocal cords and constrict the glottis. 



The arytenoid muscle assists in the closure of the aperture of the glottis. 



The inferior laryngeal nerve animates all the muscles of the larynx, with 

 the exception of the crico thyroid. 



Movements of the Vocal Cords. During respiration the move- 

 ments of the vocal cords differ from those occurring during the production 

 of voice. 



At each inspiration, the true vocal cords are widely separated, and the 

 aperture of the glottis is enlarged by the action of the crico-arytenoid 

 muscles, which rotate outward the anterior angle of the base of the aryte- 

 noid cartilages; at each expiration the larynx becomes passive; the 

 elasticity of the vocal cords returns them to their original position, and the 

 air is forced out by the elasticity of the lungs and the walls of the thorax. 



Phonation. As soon as phonation is about to be accomplished a marked 

 change in the glottis is noticed with the aid of the laryngoscope. The 

 true vocal cords suddenly become approximated and are made parallel, 

 giving to the glottis the appearance of a narrow slit, the edges of which are 

 capable of vibrating accurately and rapidly; at the same time their tension 

 is much increased. 



With the vocal cords thus prepared, the expiratory muscles force the 

 column of air into the lungs and trachea through the glottis, throwing the 

 edges of the cords into vibration. 



The //A:/; of sounds depends upon the extent to which the vocal cords 

 are made tense and the length of the aperture through which the air passes. 



