THE LARYNX 333 



other, at their posterior surfaces are certain muscles 

 called the posterior arytenoid (Fig. 105, Ar.p.). 

 There are also two sets of muscles connecting each 

 arytenoid with the cricoid, and called from their positions 

 respectively the posterior and lateral crico-ary- 

 tenoid (Fig. 105, C.a.p. C.a.l.). By the more or less 

 separate or combined action of these muscles, the 

 arytenoid cartilages, and especially the front part of these 

 cartilages and, consequently, the hinder ends of the vocal 

 cords attached to them, may be made to approach or 

 recede from each other, and thus the vocal cords rendered 

 parallel or the reverse. 



We have seen that the crico-thyroid muscle pulls the 

 thyroid cartilage down, or the cricoid cartilage up, and 

 thus increases the distance between the front of the 

 thyroid and the back of the cricoid, on which the 

 arytenoids are seated. This movement, the arytenoids 

 being fixed, must tend to pull out the vocal cords 

 lengthways, or in other words to tighten them. 



Running from the re-entering angle in the front part 

 of the thyroid, backward, to the arytenoids, alongside the 

 vocal cords (and indeed imbedded in the transverse folds, 

 of which the cords are the free edges) are two strong 

 muscles, one on each side (Fig. 105, Th.A.), called 

 thyro-arytenoid. The effect of the contraction of 

 these muscles is to pull up the thyroid cartilage after it 

 has been depressed by the crico-thyroid muscles, (or to 

 pull down the cricoid after it has been raised,) and conse- 

 quently to slacken the vocal cords. 



Thus the parallelism (6) of the vocal cords is deter- 

 mined chiefly by the relative distance from each other of 

 the arytenoid cartilages ; the tension (c) of the vocal cords 

 is determined chiefly by the upward or downward move- 

 ment of the thyroid or cricoid cartilage ; and both these 

 conditions are dependent on the action of certain muscles. 



The current of air (d) whose passage sets the cords 

 vibrating is supplied by the movemaats of expiiation, 



