Mechanism of the Larynx. 339 



glottis for breathing, when it is slightly opened. In this position 

 the vocal ligaments ab cd diverge from each other in such a manner 

 that, according to oar previous experiments, the current of the 

 breath could never excite them to vibration, whatever their longitu- 

 dinal tension might be. If the points ac be carried upwards, at 

 the same time approaching each other, so as to acquire the position 

 ac, it is manifest that this change, by increasing the distances ea, 

 fc, will, by diminishing the convexities ebafilc, draw the passage 

 into the form indicated by the dotted lines eba file, in which the 

 vocal ligaments have assumed the position proper for vibration. 

 But the motion of the arytenoid round the axis OP, (Fig. 8,) which 

 we have already described, will, in raising the vocal ligaments, 

 separate them and take them rather into the position indicated 

 by the dotted line ea". 



It only remains then to explain how the extremity V, (Fig 8,) of 

 the arytenoid may be made to rise and approach the corresponding 

 point of the other arytenoid at the same time ; for, if this is done, 

 the vocal ligaments will necessarily assume the required position. 



This motion is permitted by the sliding of the articulating surface 

 of the arytenoid upon the cricoid, already described, and is effected 

 by the thyroarytenoidei muscles. These muscles we have shewn 

 to be only employed during vocalisation, and we shall now see 

 that their peculiar structure places at the same time the arytenoids 

 in the proper position for vibration. 



The internal face of one of these muscles is seen in Fig. 3 ; 

 a bird's-eye view of the opposite one is shewn in the lower half 

 of Fig. 7 at hVf. That corresponding to Fig. 3 is removed from 

 the upper half of Fig. 7, to shew the cricoarytenoideus lateralis 

 iVX more distinctly, for a similar reason the latter muscle is re- 

 moved from the lower half to display the thyroarytenoideus kVf. 



Vol. IV. rart III. X x 



