438 



VOICE. 



[CHAP. xxxi. 



muscles, the vocal chords will be stretched by the descent of 

 the thyroid cartilage over the cricoid, a movement performed 

 by the crico-thyroid and sterno-thyroid muscles. The thyro-hyoid 

 muscle, by drawing the thyroid cartilage upwards towards the 

 hyoid bone, will assist the action of the thyro-arytenoids and 

 relax the vocal chords. 



By the action of these various muscles, the tension of the vocal 

 chords may be increased or diminished, and the size of the opening 

 of the glottis regulated at will. 



Modes of altering the aperture of the Glottis and the tension of the vocal chords. 

 Crico-thyroidei . . . stretch the vocal chords .... 



\AI i relax and P lace tlie vocal chords in 



Thyro-arytenoidei . . ^ os ft ion fop voca lization . . 



govern pitch 

 of notes. 



Crico-arytenoidei postici - 



Crico-arytenoidei 



laterales. . . . 



Arytenoidei . . . 



separate the front of the 

 arytenoid cartilages . . 



press together the front of ) 

 the arytenoid cartilages . ] 



press together the back of 

 \ the arytenoid cartilages . 



open , 



glottis govern 

 I aperture 

 close I of 

 glottis glottis. 



Nerves. The nerves of the larynx are derived from the superior 

 and inferior laryngeal branches of the vagus. Those from the 

 former are distributed to the mucous membrane, and to the crico- 

 thyroid muscles, by the external laryngeal branch ; those from the 

 latter to all the other intrinsic muscles of the larynx. Both these 

 nerves anastomose with each other beneath the mucous membrane, 

 near the arytenoid muscles. 



The inferior laryngeal or recurrent nerve has relations to the 

 innominata artery on the right side, and to the aorta on the left, 

 which often implicate it in aneurismal tumors, especially of the latter 

 artery. These, by compressing, weaken and paralyze the nerve and 

 the muscles which it supplies, and give rise to those alterations of 

 voice which are sometimes among the earliest indications of the 

 formation of an aneurism within the chest. 



Action of the Larynx and Theory of Vocalization. In the little 

 apparatus of vocal chords attached to moveable cartilages and 

 bounded by a free space above (the laryngeal ventricles) and 

 by a free space below (the tube of the trachea), in which they 

 vibrate, are found the chief conditions necessary for the produc- 

 tion of the vocal sounds over an extensive though variable range 

 of pitch. 



Although all physiologists are agreed that this apparatus is the 

 seat of the production of vocal sounds, there has, nevertheless, 

 been much difference of opinion as to whether the larynx should 



