Digastric and Styloid Muscles
The
digastric muscle (Fig. 7.35) consists of anterior and posterior bellies united
by an intermediate tendon. The posterior belly attaches to the medial surface
of the mastoid process and inclines forwards and downwards, becoming continuous
with the intermediate tendon close to the hyoid bone. This tendon pierces
stylohyoid and is anchored by a fascial sling to the hyoid bone. The anterior
belly continues forwards from the intermediate tendon to attach to the inferior
border of the mandible near the midline. Digastric elevates the hyoid bone
during swallowing and assists mylohyoid and the lateral pterygoid in depressing
the mandible when opening the mouth. The posterior belly is innervated by the
facial (VII) nerve (p. 340). The anterior belly receives its motor supply from
the mandibular (V3) division of the trigeminal nerve via the mylohyoid branch
of the inferior alveolar nerve (p. 345).
Three
muscles, stylohyoid, stylopharyngeus and styloglossus, attach to the styloid
process but diverge to reach the hyoid bone, the pharynx and the tongue,
respectively. Stylohyoid (Fig. 7.35) inclines down wards and forwards from the
posterior surface of the styloid process to attach to the body of the hyoid
bone alongside the lesser horn. The muscle or its tendon is pierced by the
intermediate tendon of digastric near the hyoid bone. Stylohyoid elevates the
hyoid and is innervated by the facial (VII) nerve.
Stylopharyngeus
(Fig. 7.36) is attached to the medial side of
the root of the styloid process and passes inferomedially on the lateral
surface of the superior pharyngeal constrictor. It enters the wall of the
pharynx between the superior and middle constrictors and blends with the other
longitudinal muscles of the pharynx. The muscle elevates the pharynx and larynx
during swallowing and is the only muscle innervated by the glossopharyngeal
(IX) nerve.
Styloglossus
(Fig. 7.36) inclines anteromedially from the tip
of the styloid process and upper end of the stylohyoid ligament and passes
between the superior and middle constrictors of the pharynx to enter the
tongue. It elevates and retracts the tongue and, in common with other muscles
of the tongue, is supplied by the hypoglossal (XII) nerve.
