234 THE ANATOMY OF THE HORSE. 



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

 cartilage 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 junction of the thyroid alee 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 facets on the upper edge of the cricoid 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 them, 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 not executed 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 

 elastic fibres connecting it to the thyroid ; but in the horse this action 

 is assisted by the hyo-epiglottideus muscle. 



the muscles of the larynx. 



Extrinsic Group. — This includes the sterno-thyroid, the thyro-hyoid, 

 and the hyo-epiglottideus. The last of these is a single muscle ; the 

 other two are double. 



The Sterno-thyroid Muscle (Fig. 30). See page 149. 



The Thyrohyoid Muscle (Figs. 30 and 31). This is a dark-coloured, 

 fleshy muscle taking origin from the thyroid cornu (heel process) of the 

 hyoid bone, and inserted into an oblique line on the outer surface of the 

 thyroid wing. 



Action. — Acting alone, the thyro-hyoid muscles would elevate the 

 larynx between the thyroid cornua of the hyoid bone ; but when they 

 act in concert with the sterno-thyroid the thyroid cartilage will be 

 steadied, and will serve as the fixed point for the crico-thyroid and 

 thyro-arytenoid muscles. 



