STRUCTURE AND ACTIONS OF THE LARYNX. 



457 



!L 



is evident that they may be rendered more or less tense by the movement of 

 the Thyroid cartilage just described ; 



being tightened by the depression of Fig. 196. 



its front upon the Cricoid cartilage, and 

 slackened by its elevation. On the 

 other hand, they may be brought into 

 more or less close apposition, by the 

 movement of the Arytenoicl cartilages ; 

 being made to approximate closely, or 

 to recede in such manner as to cause 

 the rima glottidis to assume the form 

 of a narrow V, by the revolution of 

 these cartilages. We shall now in- 3 

 quire into the actions of the muscles 

 upon the several parts of this apparatus; 

 and first into those of the larynx alone. 

 604. The depression of the front of 

 the Thyroid cartilage, and the conse- 

 quent tension of the Vocal Ligaments, 

 are occasioned by the conjoint action of 

 the Crico-thyroidei on both sides ; and 

 the chief antagonists to these are the . ,, , , 



, . i-i Bird's-eye view of larynx from above. G E H, 



J hyro-arytenoidet, which draw the the thyroid cartilage, embracing the ring of the 



front of the Thyroid back towards the cricoid r u x w, and turning upon the axis x z, 



Ary tenoid Cartilages, and thus relax the which passes through the tower horns, c, Fig. 113, 



VOCal ligaments. These tWO pairs of N F ' N F ' the arvtenoid cartilages, connected by 



i . j j ill the arytenoideus transversus ; T v, T v, the vocal 



muscles may be regarded as the prm- . 



! . ligaments; N x, the right crico-arytenoideus late- 



cipal governors of the pitch of the notes, raHs (lhe left being rem0 ved) ; v t/, the left thyro- 



whicll, as We shall hereafter See, is al- arytenoideus (the right being removed) ; N I, N I, 

 most entirely regulated by the tension the crico-arytenoidei postici; B, B, the crico-ary- 



of the ligaments; their action is as- tenoid ligaments, 

 sisted, however, by that of other muscles 



presently to be mentioned. The Arytenoid cartilages are made to diverge 

 from each other, by means of the Crico-arytenoideus posticus of each side, 

 which proceeds from their outer corner, and turns somewhat round the edge 

 of the Cricoid, to be attached to the lower part of its back ; its action is to 

 draw the outer corner backwards 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. This will be at once seen from the succeed- 

 ing diagram, in which the direction of traction of the several muscles is laid 

 down. The action of this muscle is partly antagonized by that of the Crico- 

 arytenoideus lateralis, which runs forwards and downwards from the outer 

 corner of the Arytenoid cartilage ; and its action, with that of its fellow, will 

 be to bring the anterior points of the Arytenoid cartilages into the same 

 straight line, at the same time depressing them, and thus to close the Glottis. 

 This muscle is assisted by the Arytenoideus transversus, which connects 

 the posterior faces of the Arytenoid cartilages, and which, by its contraction, 

 will draw them together. By the conjoint action, therefore, of the Crico-ary- 

 tenoideus lateralis, and of the Arytenoideus transversus, the whole of the ad- 

 jacent faces of the Arytenoid cartilages will be pressed together; and the 

 points to which the vocal ligaments are attached, will be depressed. But if 

 the Arytenoideus be put in action in conjunction with the Crico-arytenoidei 

 postici, the tendency of the latter to separate the Arytenoid cartilages being 

 antagonized by the former, its backward action only will be exerted ; and thus 

 it may be caused to aid the Crico-thyroideus in rendering tense the vocal 

 39 



