Mylohyoid and Related Structures
Suspended from the body of the mandible is a thin sheet of muscle formed by the two mylohyoid muscles. The fibres of each muscle incline downwards and medially, meeting each other in the midline to form a shallow gutter. This gutter slopes downwards and backwards and ends at the free posterior borders of the two muscles on either side of the hyoid bone.
Each mylohyoid separates the superficial tissues in the upper part of the neck from the tongue and related structures within the mouth. Below the muscle lie the anterior belly of digastric (p. 348), part of the platysma (p. 336), the facial artery and vein, and the larger superficial part of the submandibular salivary gland (Fig. 7.37). This gland curves around the posterior border of mylohyoid so that its deep part and the submandibular duct lie above the muscle. Also located on the superior aspect of mylohyoid are the sublingual glands, the tongue with its vessels and nerves and the geniohyoid muscles.
This muscle (Figs 7.37 & 7.38) takes attachment from the mylohyoid line on the inner aspect of the body of the mandible. Most of its fibres reach a midline raphe, where they interdigitate with those from the opposite side. However, the posterior fibres descend to the body of the hyoid bone. The mylohyoid muscles raise the hyoid bone and the tongue during swal-lowing; they also help to depress the mandible when the hyoid bone is fixed from below. The muscle is innervated by the mylohyoid nerve, a branch of the inferior alveolar nerve from the mandibular division of the trigeminal nerve (V3).
Above mylohyoid geniohyoid (Fig. 7.39) lies close to the midline, passing from the inferior mental spine (inferior genial tubercle) on the mandible to the body of the hyoid bone. Innervated by C1 spinal nerve fibres that are conveyed in the hypoglossal nerve, geniohyoid either elevates the hyoid or depresses the mandible.
The tongue is a muscular organ and lies mostly within the oral cavity, although its posterior part projects into the oropharynx.
The tongue is covered by mucosa, which is reflected anteriorly and laterally onto the inferior surface. Under the tip of the tongue the mucous membrane forms a midline fold, the frenulum. The upper surface of the tongue (Fig. 7.40) is divided into anterior two-thirds and posterior one-third by a V-shaped groove, the terminal sulcus. A midline depression, the foramen caecum, lies at the apex of the groove. A row of round elevations, the vallate papillae, lies just in front of the sulcus. From the mucosa of the tongue, somatic sensory and taste fibres pass centrally to reach the brain. From the anterior two-thirds of the tongue (and adjacent oral mucosa), these fibres travel in the lingual nerve (p. 345). Somatic sensory fibres continue to the brain in the mandibular (V3) division of the trigeminal nerve. However, taste fibres leave the lingual nerve and are conveyed via the chorda tympani nerve to the facial (VII) nerve (p. 345). Both taste and somatic sensory nerve fibres from the posterior third of the tongue (including the vallate papillae) are conveyed in the glossopharyngeal (IX) nerve.
The musculature of the tongue, divided into two functional halves by a fibrous septum, consists of both extrinsic and intrinsic muscles. In general, the extrinsic muscles alter the position of the tongue and the intrinsic muscles alter its shape.
The extrinsic muscles comprise styloglossus (p. 348), hyoglossus and genioglossus. Hyoglossus (Fig. 7.39) is attached to the body and greater horn of the hyoid bone and its fibres pass upwards and forwards to reach the side of the tongue. Hyoglossus depresses the tongue, particularly at the sides. Genioglossus (Figs 7.41 & 7.42) forms much of the bulk of the tongue, its fibres radiating from the superior mental spine (superior genial tubercle). The superior fibres reach the tip, while the inferior fibres pass into the posteroin-ferior part of the organ. Genioglossus draws the tongue forwards and down-wards and its anterior fibres retract the tip. Palatoglossus descends from the soft palate and enters the side of the tongue posteriorly. Although it helps to elevate the tongue, it is usually considered to be a muscle of the palate (p. 362).
The intrinsic muscles comprise interlacing longitudinal, transverse and vertical groups of fibres on either side of the midline fibrous septum. The shape of the tongue is changed by integrated contractions of the various groups.
The hypoglossal (XII) nerve (Fig. 7.39) supplies all the muscles of the tongue, both intrinsic and extrinsic. (The nerve supply to palatoglossus is described with the other muscles of the palate; p. 362.) Damage to the hypoglossal nerve causes the tongue to deviate to the injured side when protruded.
Arising from the front of the external carotid artery, the lingual artery (Fig. 7.41) loops upwards above the greater horn of the hyoid bone and continues forwards into the base of the tongue deep to hyoglos-sus. Having supplied structures in the tongue, the lingual artery terminates in the tip. Venous drainage follows a reciprocal course into the internal jugular vein.
The superficial part of the submandibular salivary gland (Fig. 7.37) lies inferior to mylohyoid and extends below the medial surface of the mandible. It is continuous around the posterior border of mylohyoid with the smaller deep part (Fig. 7.38) of the gland that lies above the muscle. The lateral surface of the superficial part of the gland is indented by the facial artery as the vessel arches forwards to reach the inferior border of the mandible (p. 337).
From the deep part of the gland the submandibular duct passes forwards and medially to open close to the midline at the base of the frenulum of the tongue. A number of small sublingual glands (Fig. 7.39) lie beneath the mucosa adjacent to the submandibular duct. These glands are drained by ducts that open either into the submandibular duct or directly into the mouth. Calculi in the submandibular duct can often be palpated through the mucosa of the floor of the mouth. Both the sublingual and submandibular glands receive postganglionic parasympathetic secretomotor fibres from the submandibular ganglion (Fig. 7.39). Preganglionic fibres reach the submandibular ganglion from the facial nerve by way of the chorda tympani and lingual nerves.
Hypoglossal (XII) nerve
The twelfth cranial nerve (Fig. 7.38) leaves the skull through the hypoglossal canal and descends between the internal jugular vein and internal carotid artery to turn forwards close to the origin of the occipital artery from the external carotid artery, where the superior root of the ansa cervicalis (p. 329) arises. The hypoglossal nerve then continues forwards across the loop of the lingual artery and passes between mylohyoid and hyoglossus to enter the tongue whose muscles it supplies. The branches given to thyrohyoid and geniohyoid are composed of C1 fibres which are distributed via the hypoglossal nerve.
The glossopharyngeal nerve (Fig. 7.41) emerges from the skull through the jugular foramen. Initially contained within the carotid sheath, the nerve curves forwards around the stylopharyngeus muscle (which it also supplies) and passes between the superior and middle constrictors of the pharynx to enter the posterior part of the tongue. The glossopharyngeal nerve conveys both somatic and taste sensation from the posterior third of the tongue, oropharynx and lateral parts of the soft palate. The nerve has an autonomic sensory branch, the carotid sinus nerve, which ascends from the carotid sinus and carotid body. In addition, a tympanic branch passes to the tympanic plexus in the middle ear. The lesser petrosal nerve arises from this plexus.