DISSECTION OF THE LARYNX. 227 



small capsular ligament, and lined by a synovial sac. The two cartilages 

 are further united by the crico-thyroid viemhrane. This is a fibro-elastic 

 structure consisting of a central and two lateral portions. The central 

 portion is triangular and fills up the space between the adjacent edges 

 of the right and left thyroid alse. It is attached by its sides to these 

 edges, while by its base it is inserted into the upper boi-der of the 

 cricoid. Each lateral portion lies imder cover of the laryngeal mucous 

 membrane, and is shaped somewhat like a quadrant, having an inferior 

 convex edge fixed to the margin of the cricoid in company with the central 

 portion, an anterior edge confounded with the central portion, and an 

 upper straight edge which is thin and free on the side of the larynx. 

 This upper edge is the true vocal cord ; and since its fibres are attached 

 in front to the angle of union of the thyroid alse, and posteriorly to the 

 projecting anterior angle of the base of the arytenoid, it is also termed 

 the thyro-arytenoid ligament. Vocal sounds are produced by the vibra- 

 tion of the vocal cords. 



Movements. — The movements between the cricoid and thyroid cartil- 

 ages take place around an imaginary horizontal axis passing through the 

 right and left crico-thyroid joints, and in these movements either cartil- 

 age may be supposed to remain fixed while the other revolves around 

 the axis. It should be observed that these movements vary the distance 

 between the angle of jiinction of the thyroid alse and the base of the 

 arytenoids, and thus vary the tension of the true vocal cords, which 

 stretch between these points. 



Union of the Cricoid and Arytenoid Cartilages, Each arytenoid 

 cartilage is articulated by the concave facet on its base to one of the 

 convex fixcets on the upper edge of the ci'icoid bezel. It is a diarthrodial 

 joint, possessing a capsular ligament and a synovial sac. 



Movements. — The arytenoid cartilage swings like a door, around a 

 vertical axis passing through the crico-arytenoid joint. When the 

 cartilage is swung outwards, the true vocal cord, which is attached to 

 the anterior angle of its base, is separated from the cord of the opposite 

 side, and the glottis is widened. The glottis is narrowed by the opposite 

 movement. 



Union of the Thyroid and Epiglottis. These cartilages are united 

 by elastic fibres passing between tliem, and forming a kind of amphi- 

 arthrosis. 



Movements. — Except during the act of deglutition, the epiglottis stands 

 erect in front of the upper aperture of the larynx. During that act 

 the cartilage is bent downwards and backwards so as to cover the 

 aperture like a lid. This movement, however, is executed not exclusively 

 at the joint between the two cartilages, but partly by a bending of the 

 whole cartilage. At the close of the act of deglutition the epiglottis 

 assumes the erect position, owing to its own elastic texture and the 



