614 Comparative Morphology of Chordates 



if the flap is bent backward over the anterior end of the larynx, the 

 laryngeal cavity is quite effectually covered. Immediately posterior 

 to the thyroid cartilage is the cricoid, which is a broad band of carti- 

 lage completely encircling the larynx. To the dorsal anterior edge of 

 the cricoid are attached a pair of very small cartilages, the arytenoids. 

 The laryngeal cartilages are tied together by ligaments which admit 

 of slight movements of one cartilage in relation to another. 



On the inside surface of each lateral wall of the thyroid cartilage 

 is an inwardly projecting fold of the mucous layer which lines the 

 larynx. The fold extends dorsoventrally, its dorsal end attached to 

 the arytenoid of that side and the ventral end meeting the ventral 

 end of the other fold at the midventral line of the inner surface of the 

 thyroid (Fig. 471, 3). These are the vocal folds, commonly called "vocal 

 cords," but inaccurately so, for they are not literally "cords." The 

 thyroid being incomplete dorsally, its lateral and ventral walls extend 

 considerably anterior to the vocal folds. The glottis, in strict sense, is 

 the slitlike opening between the vocal folds and not the much wider 

 anterior aperture of the larynx as a whole. There is usually a second 

 pair of folds situated anterior to the vocal folds and resembling them, 

 but the space between their opposite free edges is much wider than 

 the glottis. They are commonly called "false vocal cords" but, 

 better, ventricular folds, the little pocket between each one and the 

 adjacent vocal fold being known as the ventricle (Fig. 471, 1 and 2). 



A complex set of small muscles effects slight movements of one part 

 of the larynx in relation to another. There is a pair of external and 

 ventral cricothyroid muscles whose contraction pulls the thyroid 

 backward and downward in relation to the cricoid, thus increasing the 

 tension on the vocal folds. To each arytenoid is attached a dorsal 

 cricoarytenoid muscle whose contraction moves the arytenoid in 

 such a way that the slit between the vocal folds is widened, and a 

 thyroarytenoid situated on the inner surface of the larynx and acting 

 to narrow the slit or close the glottis. Several other muscles are involved 

 in the control of the vocal folds. 



Sound is produced by vibration of the vocal folds, caused by more 

 or less forcible passage of air, usually outward, through the glottis. 

 By action of appropriate muscles, the degree of tension in the vocal 

 folds may be varied, the length and thickness of the folds may be 

 slightly changed, and the width and form of the glottic slit may be 

 greatly changed. The pitch and quality of the sound depend primarily 

 on these factors. The quality, however, is largely dependent on the 

 resonant effects of vibration of air contained in all the air-filled spaces 

 connected with the larynx, especially important being the pharynx and 

 the oral and nasal cavities. The "false vocal cords" probably are not 



