RESPIRATION 277 



to the oxidation of the blood in the pulmonary capillaries. But 

 in many of the higher animals mechanisms exist whf.ch are 

 specially devoted to the utterance of sounds as their prime and 

 proper end. In man the voice-producing mechanism consists of 

 a triple series of tubes and chambers : (i) The 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 intensified, 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. 

 The arytenoids can rotate on the cricoid about a vertical axis, 

 while the cricoid can rotate on the thyroid cartilage around a 

 transverse horizontal axis. The cricoid can thus be raised 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 outwards, 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 connects 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 external 

 portion of which may to some extent cause inward rotation of 

 the vocal processes and adduction of the cords ; but the main 

 function, at least of its inner part, is to alter the tension of the 

 cords. The diagrams in Figs. 119 and 120 illustrate the action 

 of the abductors and adductors of the vocal cords. 



The crico-thyroid muscle and the deflectors of the epiglottis 

 are supplied by the superior laryngeal branch of the vagus, 

 which also contains the sensory fibres for the mucous membrane 

 of the larynx above the vocal cords. In the dog and rabbit 

 motor fibres also reach the crico-thyroid by the so-called middle 

 iaryngeal nerve which arises from the superior pharyngeai Dranch 



