Ill 



137 



arytenoids round their vertical axis, and, therefore, draw the two 

 muscular processes of the arytenoids down and back, and con- 

 sequently i'urther from the median line, and at the same time 

 raise the two vocal processes. Isolated contraction of these 

 muscles must therefore abduct the vocal cords and dilate the rima 

 glottidis ; their paralysis must, on the other hand, produce in- 

 spiratory dyspnoea owing to abnormal constriction of the rima, but 

 it does not cause appreciable disturbance of phonation. 



The constriction of the glottis is produced chiefly by the 



-tr 



FIG. 92. Larynx and hyoid bone, from the front. (Henle.) Oh, hyoid bone ; htl, lateral hyo- 

 thyroid ligament ; tr, cartilago tritica ; htm, median hyo-thyroid ligament ; ct, crico-thyroid 

 ligament ; Pp, inferior extremity of palato-pharyngeal muscle ; Th, thyro-hyoid muscle ; Cir, 

 crico-thyroid muscle divided into three bundles ; the vertical bundle on the left side has been 

 removed to show the crico-thyroid ligament ct. 



transverse arytenoid muscle, which runs between the outer 

 posterior borders of the arytenoids, and by contracting draws the 

 two bases of these cartilages towards the middle line, and their 

 mesial surfaces together, so that the intercartilaginous glottis 

 is closed. When this muscle is divided in any animal, the 

 posterior portion of the glottis remains fully open. 



Other muscles also are concerned in the active closure of the 

 glottis ; they co-operate with the transverse arytenoids to form 

 a kind of laryngeal sphincter. Among these are the thyro- 

 aryepiglottideau, and the thyro-arytenoid muscles. The two first 

 run from their point of attachment on the inner surface of the 

 thyroid obliquely backwards over the two posterior surfaces of 



