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.
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.