260 A MANUAL OF PHYSIOLOGY 



trachea, through which a blast of air is blown ; (2) the 

 larynx, with the vocal cords, by the vibrations of which 

 sound waves are set up ; and (3) the upper resonance 

 chambers, the pharynx, mouth, and nasal cavities, in which 

 the sounds produced in the larynx are modified and intensi- 

 fied, and in which independent notes and noises arise. 



The larynx is a cartilaginous box. across which are 

 stretched, from front to back, two thin and sharp-edged 

 membranes, the (true) vocal cords. In front the cords are 

 attached to the thyroid cartilage, one a little to each side of 

 the middle line ; behind they are connected to the vocal or 

 anterior processes of the pyramidal arytenoid cartilages. 

 The thyroid and the two arytenoids are mounted upon a 

 cartilaginous ring, the cricoid, on which the former can 

 rotate about a transverse horizontal axis, the latter around 

 a vertical axis. The thyroid can thus be depressed by the 

 contraction of the crico-thyroid muscle, and the vocal cords 

 stretched. By the pull of the posterior crico-arytenoid 

 muscles, attached to the external or muscular processes of 

 the arytenoid cartilages, the vocal processes are rotated out- 

 wards, the cords separated from each other or abducted, and 

 the chink between them, the rima glottidis, widened. When 

 the vocal processes are approximated by contraction of the 

 lateral crico-arytenoid muscles and the consequent forward 

 movement of the muscular processes, the vocal cords are 

 brought closer together, or adducted, and the rima is narrowed. 

 The transverse or posterior arytenoid muscle, which con- 

 nects the two arytenoid cartilages behind, also helps by its 

 contraction to narrow the glottis by shifting the cartilages 

 on their articular surfaces somewhat nearer the middle line. 

 Running in each vocal cord; and, in fact, incorporated with 

 its elastic tissue, is a muscle, the thyro-arytenoid, the ex- 

 ternal portion of which may to some extent cause inward 

 rotation of the vocal processes and adduction c : j the cords ; 

 but the main function, at least of its inner part, tS to alter 

 the tension of the cords. The diagrams :n Figs, yi and 92 

 illustrate the action of the abductors <xiid adductors of the 

 vocal cords. 



The crico-thyroid muscle and the defectors of the epi- 



