Movement of the hyoid apparatus during chewing

Abstract

THE movements of the hyoid bone and the larynx during swallowing are only well established for man^1–3. Their movements, if any, during mastication in man and in other mammals have been neglected. The hyoid is suspended from the cranium, above and behind, by the posterior suprahyoid muscles (stylohyoid and the posterior belly of digastric) and from the symphyseal region of the jaw in front by the anterior suprahyoid muscles (anterior belly of digastric, mylohyoid, geniohyoid and the more inferior fibres of genioglossus). The infrahyoid, or ‘strap’ muscles of the neck connect the hyoid with the sternum directly by the sternohyoid and indirectly through the thyroid cartilage of the larynx (thyrohyoid and sternothyroid) as well as with the scapula (omohyoid). This arrangement of the hyoid apparatus is found in all mammals, except that the digastric may not have a direct hyoid connection⁴: a situation occasionally found in man². These three groups of muscles (anterior and posterior suprahyoids and infrahyoids) are recognised, on the basis of anatomical, cinematographic⁵ and electromyographic⁶ studies of human deglutition, as acting in combination first to elevate and then to depress the hyoid and larynx in swallowing. The posterior suprahyoids and the infrahyoids are also classically regarded^1,2 as acting to “stabilise”¹ or “fix”² the hyoid so that the anterior suprahyoids, and particularly the anterior belly of digastric^6,7, can pull the symphyseal region of the jaw towards the hyoid and open the mouth. Mandibular depression follows the power stroke⁸ of every chewing cycle. The classical view of the hyoid apparatus is of a somewhat immobile bone and a group of muscles whose only active role is in swallowing. This concept may have led to the view that, since the omohyoid in man made such an uncertain contribution to hyoid movement or control of hyoid position, a “specific”¹and “speculative”² function should be attributed to this muscle; namely the tensing of the cervical fascia to assist prolonged inspiration during forced respiration. The latest edition of Gray's Anatomy² comments, “so far no useful data in regard to the hyoid musculature as a whole have been contributed by electromyographers” (page 508).