742 A TEXTBOOK OF PHYSIOLOGY 





the cricoid, the thyroid, and two arytenoids are articulated and knit 

 together by ligaments. 



The cricoid, shaped like a signet ring, articulates with the inferior 

 horns of the thyroid, and can rotate round an axis which passes 

 through these joints. Each arytenoid is jointed to the ring plate 

 of the cricoid behind by a triangular surface. This joint allows 

 movements in three planes. 



The cartilages can glide outwards and separate from each other. 

 They can turn round a vertical axis (i.e., an axis in the direction of 

 the trachea), whereby their vocal processes which project forwards 

 are turned outwards or inwards. They can move round an axis 

 which passes inwards and upwards from below and outwards, whereby 

 the vocal processes glide forwards and outwards, or backwards and 

 inwards. These movements are limited by a strong band the 

 crico-arytenoid ligament. 



The mucous membrane of the larynx is raised into folds which run 

 from before backwards. The false vocal cords do not reach so far 



FIG. 453. VIEW OF LARYNX OBTAINED BY THE LARYNGOSCOPE. 



a, Epiglottis; l>, thyroid; c, vocal cord; d, aryepiglottidean fold; e, cartilage ot 

 Wrisberg ; /, cartilage of Santorini ; g, pharynx. 



middlewards as the true. The true cords in section have the shape of 

 a three-sided prism. The upper side of each is vertical to the longi- 

 tudinal direction of the windpipe, the under surface slopes up to 

 meet this, and the edges so formed become sharp during vocalization. 

 The cords run from the vocal processes of the arytenoids to about 

 the middle of the angle formed by the two wings of the thyroid car- 

 tilage. The cords contain much elastic tissue and the fibres of the 

 thyro-arytenoideus muscle. The part of the rima between the cords 

 is called pars vocalis, and the part between the arytenoid cartilages 

 pars respiratoria. The muscular mechanism for changing tension, 

 length, and thickness, in addition to the laryngeal muscles, includes 

 the sterno-thyroid, hyo-thyroid, and laryngo-pharyngeal muscles. 

 These act indirectly, fixing the larynx and steadying it for the 

 direct action of the laryngeal muscles. 



The Posterior Crico-Arytenoid Muscles. Each arises from the 

 posterior surface of the ring plate of the cri?o : d, and is inserted into 

 the muscular process of an arytenoid. They pull these processes in- 

 wards, and so turn the vocal process outwards. Their antagonists 





