PHYSIOLOGY OF MOVEMENT. 



763 



transverse and oblique arytenoid muscles, added to the contraction of 

 the internal thyro-arytenoids. At the same time a concave margin 

 is produced in the vocal cords through the action of the crico-thyroid 

 and the posterior crico-arytenoids (Figs. 318, 319, and 320). 



Fig. 318.— Action of the Muscles of 

 the Larynx. (Beaunis.) 



The dotted lines indicate the new positions as- 

 sumed by the thyroid cartilage in the action of the 

 crico-thyroid muscles. 



1, cricoid cartilage: 2, arytenoid cartilage: 3, 

 thyroid cartilage : 4, true vocal cord ; 5, new position 

 of the thyroid cartilage ; 6, new position of the vocal 

 cords. 



Fig. 319.— Schematic Horizontal Section of 

 the Larynx. (Landois.) 

 I, position of the horizontally divided arytenoid cartilages 

 during respiration ; from their anterior processes run the con- 

 verging vocal cords. The arrows show the line of action of the 

 posterior erico-arytenoid muscles, resulting in the assumption of 

 the positions indicated by the dotted lines, II, II. 



Fig. 320.— Scheme of the Closure of the Glottis by the Thyro- 

 arytenoid Muscles. (Landois.) 

 II, n, position of the arytenoid cartilages during quiet respiration ; the arrows indicate the direction 

 of muscular traction. 1 1, the position of the arytenoid cartilages after the muscles contract. 



The thyro-arytenoid muscle is the one which principally causes 

 variations in the tension of the vocal cords and, consequently, variations 

 in the pitch of the sounds. 



When the muscles acting on the vocal cords relax the vocal cords 

 themselves likewise relax from the reduction of the extending force, and 



