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).
Geniohyoid
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.
Tongue
The
tongue is a muscular organ and lies mostly within the oral cavity, although its
posterior part projects into the oropharynx.
Surface
features
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.
Muscles
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.
Blood
supply
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.
