THE LARYNX, AND ITS ACTIONS. 299 



nearly horizontal, but partly downwards ; so that their vertical planes may be 

 made to separate from each other, and at the same time to assume a slanting 

 position. This change of place will be better understood, when the action of 

 the muscles is described. To the summit of the arytenoid cartilages are 

 attached the chordae vocctles, or Vocal Ligaments, which stretch across to the 

 front of the thyroid cartilage ; and it is upon the condition and relative situation of 

 these ligaments that their action depends. It is evident that they may be ren- 

 dered more or less tense by the movement of the thyroid cartilage just described, 

 being tightened by the depression of 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 arytenoid cartilages ; being 

 made to approximate closely, or to recede in such a manner as to cause the 

 rima glottidis to assume the form of a narrow V, by the revolution of these 

 cartilages. We shall now inquire into the actions of the muscles upon the 

 several parts of this apparatus; and first into those of the larynx alone. 



403. 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 

 Thyro-ctrytenoidei, which draw the front of the Thyroid back towards the 

 Arytenoid cartilages, and thus relax the vocal ligaments. These two pairs 

 of muscles may be regarded as the principal governors of the pitch of the 

 notes, which, as we shall hereafter see, is almost entirely regulated by the 

 tension of the ligaments ; their action is assisted, 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 around 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 subjoined dia- 

 gram, 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-arytenoi- 



Fig. 77. 



Part of Fig. 75 enlarged, to show the direction of the muscular forces, which act on the Arytenoid Car- 

 tilage ; Q N v s, the right arytenoid cartilage; T v, its vocal ligament; B R s, bundle of ligaments uniting it 

 to cricoid ; c p, projection of its axis of articulation; h g, direction of the action of the thyro-arytenoideus ; 

 N x, direction of crico-arytenoideus lateralis; N w, direction of crico-arytenoideus posticus; N T, direction 

 of arytenoideus transversus, After Willis. 



