66 THE SURGICAL ANATOMY OF THE HORSE 



The first pair are the cricn-thyroid muscles. These muscles are placed 

 one on either side the larynx. Each arises from the lateral aspect of the 

 cricoid cartilage, and the fibres extend obliquely upwards and backwards 

 to be inserted into the inferior edge of the wing of the thyroid cartilage. 

 The action of the crico-thyroid muscles is to pull the thyroid cartilage 

 downwards and so tighten the vocal cords. 



The thyro-arytenoideus muscle is also paired, and it runs from the 

 anterior portion of the deep face of the thyroid cartilage to the outer 

 aspect of the base of the arytenoid. It acts in antagonism to the 

 crico-thyroid by diminishing the tension of the vocal cords. 



The lateral crico-arytemideii muscles are placed beneath the wings of 

 the thyroid cartilage. Each is therefore deeply seated to the cor- 

 responding thyro-arytenoid muscle. The fibres of these muscles run 

 from the upper edge of the cricoid to the external tubercles and outer 

 surfaces of the arytenoid cartilages. When these muscles contract the vocal 

 cords are approximated more closely to one another by the twisting 

 of the arytenoid cartilages inwardly on their bases. 



The posterior crico-arytenoideii muscles run from the back of the cricoid 

 to the external angles of the arytenoid cartilages. During contraction of 

 these muscles the arytenoid cartilages are twisted on their bases in the 

 outward direction, and so the vocal cords are more widely separated 

 from one another. 



The arytenoideus muscle. — This is usually referred to as a single 

 unpaired muscle. Strictly, however, there are two muscles which meet 

 in a median fibrous raphe. From this raphe each muscle runs down- 

 wards and outwards, to be inserted into the posterior aspect of the 

 arytenoid cartilage of its own side. When these little muscles contract 

 the arytenoid cartilages are drawn closer together, and so the edges of 

 the cords are approximated and made parallel. 



Mucous Membrane of the Larynx. — The mucous membrane which 

 lines the pharynx envelops the opening into the larynx and then passes 

 through the opening into the laryngeal chamber. It then covers the 



