Origin: Inner surface of the thyroid cartilage (anterior aspect)
Insertion: Anterior surface of arytenoid cartilage
Nerve: Recurrent laryngeal branch of the vagus
Action: Thickens the vocal folds and decreases length; also helps helps to adduct the vocal folds during speech
The Thyreoarytaenoideus (Thyroarytenoid) is a broad, thin, muscle which lies parallel with and lateral to the vocal fold, and supports the wall of the ventricle and its appendix. It arises in front from the lower half of the angle of the thyroid cartilage, and from the middle cricothyroid ligament. Its fibers pass backward and lateralward, to beinserted into the base and anterior surface of the arytenoid cartilage. The lower and deeper fibers of the muscle can be differentiated as a triangular band which is inserted into the vocal process of the arytenoid cartilage, and into the adjacent portion of its anterior surface; it is termed the Vocalis, and lies parallel with the vocal ligament, to which it is adherent.
A considerable number of the fibers of the Thyreoarytaenoideus are prolonged into the aryepiglottic fold, where some of them become lost, while others are continued to the margin of the epiglottis. They have received a distinctive name, Thyreoepiglotticus, and are sometimes described as a separate muscle. A few fibers extend along the wall of the ventricle from the lateral wall of the arytenoid cartilage to the side of the epiglottis and constitute the Ventricularis muscle.
Action: The Thyreoarytœnoidei, consisting of two parts having different attachments and different directions, are rather complicated as regards their action. Their main use is to draw the arytenoid cartilages forward toward the thyroid, and thus shorten and relax the vocal folds. But, owing to the connection of the deeper portion with the vocal fold, this part, if acting separately, is supposed to modify its elasticity and tension, while the lateral portion rotates the arytenoid cartilage inward, and thus narrows the rima glottidis by bringing the two vocal folds together.