488 MANUAL OF PHYSIOLOGY. 



that one surface looks inward, the second backward, and the 

 third forward and outward, while the inferior surface rides on 

 the cricoid. One point, therefore, looks forward, and to it is 

 attached the vocal cord on each side, hence it has been called 

 the vocal process. The apex, which looks outward and back- 

 ward, gives attachment to some of the intrinsic muscles, and 

 hence has been called the muscular process. 



The thyroid cartilage is connected with the cricoid below, and 

 to the hyoid bone above by ligaments and tough membranes, 

 which hold the parts together, fill in the intervals, and complete 

 the skeleton of the larynx. 



The vocal cords are composed of small strands of elastic 

 tissue, which are stretched between the anterior processes of the 

 arytenoid cartilages and the inferior part of the thyroid, where 

 they are attached side by side to the posterior surface of the 

 angle formed by the junction of the two lateral parts or alse of 

 the thyroid. The mucous membrane which lines the larynx is 

 thin, and closely adherent over the vocal cords. The surface of 

 the laryngeal cavity is smooth and even, the lining membrane 

 passing over the cartilages and muscles so as to obliterate all 

 ridges except the vocal cords and two others, less sharply 

 defined, called the false vocal cords, which lie parallel to and 

 above the true vibrating cords. Between these is the cavity 

 known as the ventricle of the larynx. 



MECHANISM OF VOCALIZATION. 



Taking the thyroid cartilage as the fixed base, the cricoid and 

 arytenoid cartilages undergo movements which bring about two 

 distinct sets of changes in the glottis and its elastic edges, namely, 

 (1) widening and narrowing the opening ; (2) stretching and re- 

 laxing of the vocal cords. During ordinary respiration the glottis 

 remains about half open, the muscles being in a state of relaxa- 

 tion (B'). During forced inspiration the glottis is widely dilated 

 by muscular action (C'). If an irritating gas be inspired, the 

 glottis is tightly closed by a spasmodic action of certain muscles, 

 so that the true vocal cords act as a kind of valve. 



During vocalization, the glottis is formed into a narrow chink 



