Parotid Gland
The parotid is the largest of the salivary
glands and is pyramidal in shape (Fig. 7.25). Its
base faces laterally between the anterior border of sternocleidomastoid and the
ramus of the mandible, while its apex lies deeply against the styloid process.
The gland extends upwards between the external acoustic meatus and the
temporomandibular joint, movements of which may be painful when the parotid is
inflamed.
The
parotid gland is deeply indented in front by the mandible, masseter and medial
pterygoid. Deep to the gland are the infratemporal fossa and the styloid
apparatus, the latter separating it from the carotid sheath and the pharyngeal
wall.
The
investing fascia of the neck splits into two layers to enclose the gland (p.
324). The superficial layer attaches above to the zygomatic arch, while
anteriorly it merges with the tissues of the cheek. The deep layer is attached
to the tympanic part of the temporal bone and is thickened between the styloid
process and the angle of the mandible to form the stylomandibular ligament.
Swelling in the parotid is particularly painful because of the toughness of the
surrounding fascia and its position between the mandible in front and the
temporal bone behind.
The
parotid duct (Fig. 7.26) passes
forwards across the surface of masseter, turns medially and pierces buccinator
to open obliquely into the
vestibule of the
mouth opposite the upper second molar tooth. This oblique
passage through the mucosa compresses the duct when intraoral pressure is
raised. When masseter contracts, the duct may be palpated over the anterior
edge of the muscle. An accessory part of the gland may lie alongside the duct.
Of
these, the most superficial are branches of the facial (VII) nerve that run
forwards to the face and pass superficial to the retromandibular vein and its
tributaries. The deepest is the termination of the external carotid artery.
The facial
nerve emerges from the skull through the stylomastoid foramen and turns
forwards to enter the posterior surface of the parotid gland. Before
penetrating the gland, it gives two branches. The posterior auricular branch
passes behind the external acoustic meatus to supply occipitalis and the auricular
muscles. The other supplies the posterior belly of digastric and stylohyoid.
Within the gland, the facial nerve forms a plexus from which five groups of
branches (Fig. 7.26) emerge at the periphery of the gland to supply the muscles
of facial expression (p. 335). This plexus divides the gland into superficial
and deep parts. Tumours confined to one of these parts only can be removed
without damage to the facial nerve.
Retromandibular
vein
Within
the parotid gland, the superficial temporal and maxillary veins unite to form
the retromandibular vein (Fig. 7.27).
Inferiorly, this short vein terminates
as anterior and
posterior divisions, which escape
from the gland. The anterior division joins the facial vein, which drains into
the internal jugular vein. The posterior division unites with the posterior
auricular vein to form the external jugular vein (p. 326).
Termination
of external carotid artery
Entering
the parotid from below, the external carotid artery ascends through the gland
and at the level of the neck of the mandible divides into the maxillary and
superficial temporal arteries (Fig. 7.28). The
former runs anterosuperiorly to leave the gland and enter the infratemporal
fossa. The superficial temporal artery continues upwards between the external
acoustic meatus and the temporomandibular joint to supply the temple. Its
pulsation can be felt just above the joint.
Several
lymph nodes lie just under the fascia covering the parotid gland or within the
gland itself.
The
gland receives its vascular supply from the vessels traversing it.
Parasympathetic secretomotor fibres follow a tortuous route to the gland. The
preganglionic fibres arise from the inferior salivatory nucleus in the
brainstem and pass in the tympanic branch of the glossopharyngeal (IX) nerve to
the tympanic plexus in the middle ear, leaving the skull in the lesser petrosal
nerve to synapse in the otic ganglion (p. 345). Postganglionic parasympathetic fibres
travel to the gland in the auriculotemporal branch of the mandibular (V3)
division. By contrast, postganglionic sympathetic nerves are conveyed in the
plexus accompanying the external carotid artery.
