SOUND PRODUCTION AND SPEECH 



743 



are the lateral crico-arytenoid muscles; each arises from the 

 inner surface of the ring of the cricoid, and is inserted on the 

 muscular processes of an arytenoid. They close the chink of the 

 glottis. 



The arytenoid muscles, transverse and oblique, bring the two 

 arytenoid cartilages together. If these muscles are slack, the 

 lateral and posterior crico-arytenoids, acting together, widen the 

 chink. 



The crico-thyroids are the chief tighteners of the cords. The 

 fibres arise from the outer surface of the ring on either side, and run 

 upwards and backwards to the thyroid. They pull the front part of 

 the ring up to the thyroid and depress the ring-plate ; and this move- 

 ment, carrying the arytenoids with it, lengthens the distance between 



Th.O. 



Th.A. 



V.Pr, 



A.C 



M.Pr.- 



C.A.L.. 



O.A.P. 



FIG. 45-i. SCHEME OF LARYNGEAL MUSCLES. (Parsons and Wright.) 



Th.C., Thyroid cartilage; A.C., arytenoid cartilage; Th.A., thyro-arytenoideus; 

 O.A.L., crico-arytenoideus lateralis; C.A.P., crico-arytenoideus posticus; A., 

 arytenoideus ; V.Pr., vocal process of arytenoid; H.Pr.. muscular process of 

 arytenoid . 



the vocal processes and the thyroid. The antagonists are the thyro- 

 arytenoids, which run from the anterior angle of the thyroid to the 

 vocal process, some of the fibres beginning and ending in the elastic 

 tissue of the cords. These muscles can thicken the cord and press 

 its edge towards the middle line. Acting with the crico-thyroid, they 

 increase the tension of the cords. 



The tension of the cords has been measured by passing the blades 

 of a scissor-like spring gauge between them. The greatest tension is 

 produced by the combined action of the crico-thyroid, posterior 

 crico-arytenoid, and thyro-arytenoid i.e., the pull is said to be equal 

 to that of a kilogramme. The thyro-arytenoids also can vary the 

 tension of different parts of the cords. 



The whole length of the cords, including the vocal processes, 

 vibrates in deep notes, and the tension is raised as the pitch rises. 

 For the higher notes, the vocal processes are pressed together and 

 the shortened cords alone used. 



In using the chest or normal register, the chink is narrow and long, 



