REGULATION OF THE APERTURE OF THE GLOTTIS. 555 



culated to the sides of the Cricoid by its lower horns, round the extre- 

 mities of which it may be considered to rotate, as on a pivot. In this 

 manner, the front of the Thyroid cartilage may be lifted up, or de- 

 pressed, by the muscles which act upon it ; whilst the position of its 

 posterior part is but little changed. Upon the upper surface of the 

 back of the Cricoid cartilage, are seated the two small Arytenoid carti- 

 lages (N F) ; these are so tied to the cricoid by a bundle of strong liga- 

 ments (B B), as to have a sort of rotation upon an articulating surface, 

 which enables them to be approximated or separated from each other, 

 their inner edges being nearly parallel in the first case, but slanting 

 away from each other in the second. To the summit of these cartilages 

 are attached the Chordce vocales, or vocal ligaments (T u) composed of 

 yellow fibrous or elastic tissue. These stretch across to the front of 

 the Thyroid cartilage ; and it is upon their condition and relative situ- 

 ation, that the absence or the production of vocal tones, and all their 

 modifications of pitch, depend. They are rendered tense by the de- 

 pression of the front of the Thyroid cartilage, and relaxed by its ele- 

 vation ; by which action the pitch of the tones is regulated. But for 

 the production of any vocal tones whatever, they must be brought into 

 a nearly parallel condition, by the mutual approximation of the points 

 of the arytenoid cartilages to which they are attached ; whilst in the 

 intervals of vocalization, these are separated, and the rima glottidis, or 

 fissure between the chordae vocales, assumes the form of a narrow V, 

 with its point directed backwards. 



975. Thus there are two sets of movements concerned in the act of 

 vocalization ; the regulation of the relative position of the Vocal 

 Cords, which is effected by the movements of the Arytenoid cartilages ; 

 and the regulation of their tension, which is determined by the 

 movements of the Thyroid cartilage. The Arytenoid cartilages are 

 made to diverge from one another by means of the Orico-arytenoidei 

 postici of the two sides (N Z, N /), which proceed from their outer cor- 

 ners and turn somewhat round the edge of the Cricoid, to be attached 

 to the lower part of its back ; their action is to draw the outer corners 

 of the Arytenoid cartilages outwards and downwards, so that the 

 points to which the vocal ligaments are attached are separated from 

 one another, and the rima glottidis is thrown open. The action of 

 these muscles is antagonized by that of the Arytenoideus transversus, 

 which draws together the Arytenoid cartilages; and by that of the 

 Orico-arytenoidei laterales of the two sides (N x), which run forwards 

 and downwards from the outer corners of the Arytenoid cartilages, and 

 tend by their contraction to bring together their anterior points, to 

 which the Vocal ligaments are attached. The depression of the front 

 of the Thyroid cartilage, and the consequent tension of the Vocal liga- 

 ments, is occasioned by the conjoint action of the Orico-thyroidei of 

 the two sides, which occasions the Thyroid and Cricoid cartilages to 

 rotate, the one upon the other, at the articulation formed by the infe- 

 rior cornua of the former ; and this action will be assisted by the 

 Sterno-thyroidei, which tend to depress the front of the Thyroid carti- 

 lage, by pulling from a fixed point below. On the other hand, the 

 elevation of the front of the Thyroid cartilage, and the relaxation of 



