1076 THE EESPIEATOEY SYSTEM. 



muscle of the pharynx, and sometimes shows a certain amount of continuity 

 with it. 



The general action of the crico-thyreoideus is to render the vocal fold tense, 

 as is more fully set forth in the paragraph dealing with the actions of the intrinsic 

 laryngeal muscles as a whole. 



The innervation is derived from the external ramus of the superior laryngeal 

 nerve. 



The Mm. Vocales (O.T. Internal Thyreo-arytsenoid Muscles). The vocalis 

 muscle of each side is a triangular, somewhat prismatic muscle, which forms with 

 the thyreo-arytaenoideus a common muscular mass separable only with difficulty 

 into its two constituent portions. The m. vocalis lies within the vocal fold, closely 

 applied to the lateral aspect of the ligamentum vocale, and receives its prismatic 

 form from this adaptation. Of its three surfaces, the medial lies adjacent to the 

 ligamentum vocale and the conus elasticus ; the upper surface is -bounded by the 

 labium vocale and the entrance to the ventricle of the larynx ; whilst the lateral 

 surface is bounded by, and is in contact with, the thyreo-arytaenoideus. 



The in. vocalis arises from the inferior part of the angular depression between 

 the two laminae of the thyreoid cartilage, and also from the corresponding vocal fold, 

 whence the fibres run dorsally to be inserted into the lateral aspect of the vocal 

 process and the depression on the ventro-lateral surface of the arytaenoid cartilage. 

 The greater thickness of the m. vocalis dorsally than ventrally is due to the fact 

 that whilst all its fibres are attached dorsally to the arytaenoid cartilage, only a 

 certain proportion obtain attachment ventrally to the thyreoid cartilage. The 

 remainder are attached at irregular intervals to the lateral margin of the vocal 

 fold itself. 



It follows that the action of the muscle is to draw the vocal process of the 

 arytaenoid cartilage ventrally, thereby relaxing the vocal fold and ligament. 



The innervation is derived from the anterior ramus of the inferior laryngeal 

 nerve. 



The m. ventricularis is composed of several bundles of fibres, visible under 

 the microscope, which pass, in a vertical direction, from the lateral edge of the 

 arytaenoid cartilage to the lateral margin of the cartilage of the epiglottis. 

 There are also intermingled with it fibres from the thyreo-arytaenoideus and 

 aryepiglotticus. 



Action of the Intrinsic Laryngeal Muscles. By the action of the intrinsic 

 laryngeal muscles the position and tension of the vocal folds are so influenced that, 

 during the passage of air through the larynx, the tone and the pitch of the voice is 

 determined. Tension of the vocal folds is produced by the contraction of the two 

 crico-thyreoid muscles. The straight portions of the muscles pull the inferior border of 

 the thyreoid cartilage downwards, whilst the oblique portions, through their insertion into 

 the inferior cornua, draw the thyreoid cartilage ventrally, thereby increasing the distance 

 between the angle of the thyreoid cartilage and the vocal processes of the arytsenoid 

 cartilages. When the crico-thyreoid muscles cease to contract, the relaxation of the cords 

 is brought about by the elasticity of the ligaments. The thyreo-arytaenoid muscles must 

 be regarded as antagonistic to the crico-thyreoid muscles. When they contract they 

 approximate the angle of the thyreoid cartilage to the arytsenoid cartilages, and still further 

 relax the vocal folds ; and when they cease to act the elasticity of the ligaments of 

 the larynx again restores the state of equilibrium. They also act as constrictors of the 

 supraglottic region of the larynx. The vocales muscles, by the attachments of their 

 fibres into the vocal folds, may tighten portions of these folds, but their main action is 

 relaxation of the vocal folds and ligaments. 



The width of the rima glottidis is regulated by the arytaenoideus transversus, 

 which draws together the two arytsenoid cartilages, and this may be done so effectually 

 that the medial surfaces of these cartilages come into contact ; the pars intercartilaginea 

 of the rima glottidis is thus completely closed. The crico-aryteenoidei, laterales 

 and posteriores, also modify the width of the rima glottidis. When they act together 

 they assist the aryteenoideus transversus in closing the glottis, but when they act 

 independently they are antagonistic muscles. Thus the crico-arytsenoidei posteriores 

 draw the muscular processes of the arytsenoid cartilages dorsally and medially, and 

 swing the vocal processes and the vocal folds laterally, and thereby open the rima. The 



