928 OF THE VOICE AND SPEECH. 



Govern the Pitch of the Notes. 



f PUT TTTV f De P ress the front of the Thyroid cartilage on the Cri- 



I { s~o T^IDEI } ...... coid ' and stretch the vocal lights; assisted by the 



I Arytenoideus and Crico-arytenoidei postici. 

 Elevate the front of the Thyroid cartilage, and draw 



the 



Govern the Aperture of the Glottis. 

 "S CRICO-ARYTENOIDEI POSTICI ................ Open the Glottis. 



f CRICO-ARYTENOIDEI LATERALES 1 f Press together the inner edges of the Ary- 



< \ ARYTENOIDEUS TRANS VERSUS / " " \ tenoid cartilages, and close the Glottis. 



930. The muscles which stretch or relax the Yocal ligaments are entirely 

 concerned in the production of Voice ; those which govern the aperture of the 

 Glottis have important functions in connection with the Respiratory actions in 

 general, and stand as guards (so to speak) at the entrance to the lungs. Their 

 separate actions are easily made evident. In the ordinary condition of rest, it 

 seems probable that the Arytenoid cartilages are considerably separated from 

 each other ; so as to cause a wide opening to intervene between their inner 

 faces, and between the vocal ligaments, through which the air freely pass*es ; 

 and the vocal ligaments are at the same time in a state of complete relaxation. 

 We can close the aperture of the Glottis by an exertion of the will, either 

 during inspiration or expiration; and its closure by an automatic impulse forms 

 part of the acts of Coughing and Sneezing ( 555), besides giving rise to those 

 more prolonged impediments to the ingress and egress of air which have been 

 already noticed as resulting from disordered states of the Nervous system 

 ( 849, xvii). With these actions, the muscles which regulate the tension of 

 the vocal ligaments have nothing to do ; and we have seen that they are per- 

 formed by the instrumentality of the Pneumogastric or proper Respiratory nerve 

 ( 718). A slight examination of the recent Larynx is sufficient to make it 

 evident, that when once the borders of the rima glottidis are brought together 

 by muscular action, the effect of strong aerial pressure on either side (whether 

 produced by an expulsory blast from below, or by a strong inspiratory effort, 

 occasioning a partial vacuum below, and consequently an increased pressure 

 above), will be to force them into closer apposition. In order to produce a 

 Vocal sound, it is not sufficient to put the ligaments into a state of tension ; 

 they must also be brought nearer to each other. That the aperture of the 

 glottis is greatly narrcwed during the production of sounds, is easily made evi- 

 dent to one's self, by comparing the time occupied by an ordinary expiration, with 

 that required for the passage of the same quantity of air during the sustenance 

 of a vocal tone. Further, the size of the aperture is made to vary in accordance 

 with the note which is being produced ; of this, too, any one may convince 

 himself, by comparing the times during which he can hold out a low and a high 

 note ; from which it will appear, that the aperture of the glottis is so much 

 narrowed in producing a high note, as to permit a much less rapid passage of 

 air than is allowed when a low one is sounded. This adjustment of the aperture 

 to the tension of the vocal ligaments is a necessary condition for the production 

 of a clear and definite tone. It further appears that, in the narrowing of the 

 glottis which is requisite to bring the vocal ligaments into the necessary approxi- 

 mation, the upper points of the Arytenoid cartilages are caused to approximate, 

 not only by being made to rotate horizontally towards each other, but also by a 



