546 MUSCLES OF THE LARYNX. 



The Thyro-epiglottideus appears to be formed by the upper fibres of 

 the thyro-arytenoideus muscle: they spread out upon the external 

 surface of the sacculus laryngis and in the aryteno-epiglottidean fold 

 of mucous membrane, in which they are lost ; a few of the anterior 

 fibres being continued onwards to the side of the epiglottis. 



The Aryieno-epiglottideus superior consists of a few scattered fibres, 

 which pass forwards in the fold of mucous membrane forming the 

 lateral boundary of the entrance into the larynx, from the apex of the 

 arytenoid cartilage to the side of the epiglottis. 



The Aryteno-epiglottideus inferior. This muscle, described by Mr. 

 Hilton, is very important in relation to the sacculus laryngis, with 

 which it is closely connected. It may be found by raising the mucous 

 membrane immediately above the ventricle of the larynx. It arises 

 by a narrow and fibrous origin from the arytenoid cartilage, just above 

 the attachment of the chorda vocalis ; and passing forwards, and a 

 little upwards, expands over the upper half, or two-thirds of the sac- 

 culus laryngis, and is inserted by a broad attachment into the side of 

 the epiglottis. 



Actions. From a careful investigation of the muscles of the larynx 

 Mr. Bishop* concludes that the crico-arytenoidei postici open the 

 glottis, while all the rest close it. The arytenoideus approximates the 

 arytenoid cartilages posteriorly, and the crico-arytenoidei and thyro- 

 arytenoidei anteriorly ; the latter, moreover, close the glottis mesially. 

 The crico-thyroidei are tensors of the chordae vocales, and these 

 muscles, together with the thyro-arytenoidei regulate the tension, po- 

 sition, and vibrating length of the vocal cords. 



The crico-thyroid muscles effect the tension of the chordae vocales by 

 rotating the cricoid on the inferior cornua of the thyroid; by this ac- 

 tion the anterior portion is drawn upwards and made to approximate 

 the inferior border of the thyroid, while the posterior and superior 

 border of the cricoid, together with the arytenoid cartilages, is carried 

 backwards. The crico-arytenoidei postici separate the chordae vocales 

 by drawing the arytenoid cartilages outwards and downwards. The 

 crico-arytenoidei laterales, by drawing the outer angles of the aryte- 

 noid cartilages forwards, approximate the anterior angles to which the 

 chordae vocales are attached. The thyro-arytenoidei draw the aryte- 

 noid cartilages forwards, and, by their connection with the chorda vo- 

 cales, act upon the whole length of those cords. 



The thyro-epiglottideus acts principally by compressing the glands 

 of the sacculus laryngis and the sac itself: by its attachment to the 

 epiglottis it would act feebly upon that valve. The aryteno-epiglot- 

 tideus superior serves to keep the mucous membrane of the sides of the 

 opening of the glottis tense, when the larynx is drawn upwards, and 

 the opening closed by the epiglottis. Of the aryteno-epiglottideus, 

 the " functions appear to be," writes Mr. Hilton, " to compress the 

 subjacent glands which open into the pouch ; to diminish the capacity 



* Cyclopaedia of Anatomy and Physiology. Article, Larynx. 



