10() OF THE VOICE AMU SPEECH. 



voice, for in blowing into the trachea and larynx of an animal a 

 slight sound only is heard, unless you approximate the arytenoid 

 cartilages to each other, when a sound somewhat analogous to 

 the voice of the animal will be produced, and more acute in pro- 

 portion to their approximation, and it will be seen, at the same 

 time, that the sound is caused chiefly by the vibrations of the 

 inferior ligaments of the glottis. Again, an opening below the 

 inferior ligaments destroys the voice, while one above it, even 

 through the epiglottis, superior ligaments, and arytenoid carti- 

 lages, has no such effect. In grave tones, the whole length of 

 the inferior ligaments may be seen in a dog to vibrate ; in more 

 acute, the posterior part only j and in very acute, merely the 

 arytenoid extremity, the opening of the glottis being of course 

 lessened in the same proportion. These circumstances depend 

 upon the thyro-arytenoid muscles, which run on each side from 

 the arytenoid to the thyroid cartilage and form the lips of the 

 glottis (and indeed also the superior ligaments,) covered by an 

 aponeurosis, and this again by the mucous membrane. In pro- 

 portion as these contract, they become shorter and more tense, 

 and lessen the mouth of the glottis ; but the complete closure 

 of the glottis at the back part is effected by the arytenoid muscle, 

 which connects the two arytenoid cartilages. As all these are 

 voluntary muscles, the division of their nerves destroys the 

 voice. The division of the recurrents, which supply the thyro- 

 arytenoid muscles, is sufficient for this purpose ; but, in some 

 instances, a sound still remains similar to Avhat may be pro- 

 duced after death by blowing through the larynx after approxi- 

 mating the arytenoid cartilages, and must be owing to the action 

 of the arytenoid muscle, which is supplied not by the recurrent 

 but by the superior laryngeal nerves. As this muscle is the chief 

 means of contracting the posterior part of the glottis and pro- 

 ducing the most acute sounds, the division of the superior laryn- 

 geal nerves destroys almost all acute sounds and renders the 

 voice grave. When the division of the recurrent nerves proves 



