340 Mr Willis on the 



In Fig. 3 the arytenoid and its attached vocal ligament and 

 muscle are in the vibrating position. When the arytenoid is in 

 the position corresponding to Fig. 5, the point F is considerably 

 below the line EH- Hence when the thyroarytenoid muscle 

 is brought into action, its fibres, which lie on this face parallel 

 to the vocal ligament, tend of course to bring the points EFB 

 into a straight line and hence raise the point F. This, by in- 

 ducing a rotation round the axis OF, would separate the liga- 

 ments were it not counteracted by the direction of the fibres of 

 the lower portion of the muscle, which arising from about m, near 

 the median plane are attached to the arytenoid at a much greater 

 distance from it ; this may be seen clearly in Fig. 7. They, 

 therefore, draw the point N (Fig. 7, 8) of the arytenoid towards 

 the median plane producing the sliding motion so often alluded 

 to, whilst at the same time the upper fibres of the muscle, which 

 are not parallel to the lower, maintain the upper part of the carti- 

 lages in their due position with their points separated, so as to 

 part the upper ligaments of the glottis and keep them out of the 

 way of the current of air. 



This may be elucidated by considering the muscle when in the 

 position corresponding to Fig. 5 as a very loosely twisted rope, 

 which when brought into action tends by untwisting itself to bring 

 its fibres into parallelism, and therefore to communicate a rotatory 

 motion to the attached arytenoid, which, combined with its articu- 

 lation to the cricoid, places it in the exact position required for the 

 vibration of its vocal ligament, which then assumes the form Fig. 1. 



As the arytenoids are hidden from our sight, and cannot be 

 traced externally as the other cartilages can, it is plain that the 

 whole account I have given of their motions must be considered 

 as depending entirely upon induction. 



