ACTION OF THE VOCAL COEDS. 353 



in man may be gathered by supposing it to be composed of three por- 

 tions, the trachea, the larynx, and the mouth. The trachea Description of 

 is the tube by which air is brought from the lungs and de- the lar J nx - 

 livered into the larynx, which is a superposed structure, arranged upon 

 the cricoid cartilage, on which is articulated the thyroid cartilage by its 

 lower horns, around which a certain degree of rotation can be accomplish- 

 ed, so that the front of the thyroid may be elevated or depressed with a 

 kind of bowing motion. Posteriorly, on the cricoid cartilage are placed 

 the arytenoid cartilages, which can be approached or separated from each 

 other, and from their summits pass to the front of the thyroid cartilage 

 the inferior laryngeal ligaments or vocal cords. These constitute the 

 essential organ of sound. The thyroid cartilage, by its motions, can de- 

 termine the strain put upon them, and the arytenoids can either bring 

 them into parallelism, or place them at an acute angle. The chink or fis- 

 sure between them is the rima glottidis : its figure and width vary with 

 the recession or approximation of the vocal cords, which, as the air passes 

 by them, are thrown into vibration in the same manner as the reed in mu- 

 sical instruments. The epiglottis cartilage, which is above, guards the 

 passage, and may also be supposed, by its descent, to deaden the sounds. 



The slowness or rapidity of the vibration is dependent on the stretch 

 of the vocal cords. The manner in which various degrees Regulation of 

 of tension can be given to the cords is readily understood by the vocai cords, 

 considering their attachments. In front, as we have said, they are fast- 

 ened to the thyroid cartilage, posteriorly to the arytenoids. When the 

 thyroid cartilage executes a bowing motion forward, the vocal cords are 

 put upon the stretch, and similar variations of their tension and also of 

 their position can be given by the movements of the arytenoid cartilages 

 behind. When the air is moving in and out without giving rise to any 

 sound, the chink of the glottis is angular, its point being forward, and 

 from that the cords diverge posteriorly. For the production of sound, 

 the cords must be brought parallel, or even inclining toward each other. 

 If they incline away from each other, no sound will be produced. The 

 pitch of the note will be determined by the stretch of the cords, and this, 

 in its turn, will be determined by the contraction of the vocal muscles. 

 The crico-thyroid and sterno-thyroid bow the front of the thyroid cartil- 

 age down, the thyro-arytenoid and thyro-hyoid carry it back ; the for- 

 mer therefore stretch the cords, and the latter relax them. The opening 

 of the glottis is likewise determined by other muscles, the posterior cri- 

 co-arytenoid dilating it, and the lateral crico-arytenoid and the transverse 

 arytenoid closing it. 



Fig. 181, p. 354, is the larynx, seen in profile : <z, a, half of the hyoid 

 bone ; b, thyroid cartilage, cut ; c, thyro-hyoid membrane ; <#, cricoid 

 cartilage ; e, trachea ; f, oesophagus ; g, epiglottis ; A, great horn of the 



Z 



