1288 A MANUAL OF ANATOMY 



Nerve-supply. — ^The external lar5mgeal branch of the superior 

 laryngeal nerve. 



The fibres are directed upwards and backwards in a diverging 

 manner, the posterior being horizontal and the anterior oblique. 

 These two sets of fibres are usually distinct. 



Action. — To approximate the front parts of the cricoid and 

 thyroid cartilages in the following manner : the posterior horizontal 

 fibres draw the cricoid cartilage backwards, and the anterior oblique 

 fibres elevate the anterior part of the cricoid cartilage, thus impart- 

 ing a swinging movement to the cartilage. As a result of this 

 movement the deep posterior part of the cricoid cartilage, with the 

 arytenoid cartilages on its upper border, are depressed. In this 

 manner the vocal cords are put upon the stretch, so that the muscles 

 are tensors of the cords. 



Between the two muscles anteriorly there is a triangular interval 

 about I inch wide, in which the central portion of the crico-thyroid 

 membrane is visible, this portion being crossed at its centre by the 

 crico-thyroid arch of arteries. In this region, just above the 

 cricoid cartilage, laryngotomy may be performed. 



Posterior Crico-arytenoid — Origin. — The posterior surface of the 

 cricoid cartilage on one side of the median vertical ridge. 



Insertion. — ^The posterior aspect of the muscular process or 

 external angle of the base of the arytenoid cartilage. 



Nerve-supply. — The recurrent laryngeal nerve. 



The fibres of the muscle are directed upwards and outwards, 

 the highest being short and nearly horizontal, the middle being 

 oblique, and the lowest almost vertical. 



Action. — ^To draw the muscular process of the arytenoid cartilage 

 backwards and inwards, thereby swinging the vocal process or 

 anterior angle outwards, the result of which is to open or widen 

 the rima glottidis. The muscle is, therefore, a dilator of the rima 

 glottidis. 



The muscle is separated from its fellow by the median vertical 

 ridge on the back of the cricoid cartilage. 



Lateral Crico-arytenoid — Origin. — ^The lateral portion of the 

 cricord cartilage along its upper sloping border, extending as far 

 back as the crico-arytenoid joint. 



Insertion. — ^The anterior aspect of the muscular process of the 

 arytenoid cartilage, and the adjacent portion of the antero- 

 external surface. 



Nerve-supply. — ^The recurrent laryngeal nerve. 



The fibres of the muscle are directed backwards and upwards. 



Action. — ^To draw the muscular process of the arytenoid cartilage 

 forwards and inwards, thereby swinging the vocal process or 

 anterior angle inwards, the result of which is to narrow the rima 

 glottidis. The two muscles, therefore, approximate and relax the 

 true vocal cords, and they come into action in vocalization. When 

 the posterior and lateral crico-arytenoid muscles act together they 

 assist the arytenoid muscle in approximating the arytenoid carti- 



