The thyroarytenoid muscle is a complex muscle which forms the main muscular bulk of the vocal fold. Some consider it consists of two parts the inner vocalis muscle and the outer lateral thyroarytenoid muscle. The thyromuscularis part arises from the lower half of the angle of the thyroid cartilage and runs posteriorly to insert into the base of the arytenoid cartilage. It is innervated by ‘fast-twitch’ nerve fibres which are necessary for coughing and rapid protection of the airway. The vocalis part which arises more from the vocal ligament and inserts into the vocal process of the arytenoid. There is no good evidence the vocalis is a separate muscle but it is innervated by different types of nerve fibres (‘slow-twitch’ fibres) which are found when the muscles need to contract in a sustained and very accurate manner as is required for speech and singing. The thyroarytenoid muscle appears to have evolved in function to allow the larynx to produce  precise changes in shape, position and tensioning for speech and singing as well as continuing with its original role of protection of the lower airways.