Last's Anatomy: Regional and Applied

Part seven. Parotid region

The part of the face in front of the ear and below the zygomatic arch is the parotid region. The principal features are the parotid gland and the masseter muscle.

Masseter

This quadrilateral muscle of mastication arises from the lower border of the zygomatic arch and is inserted into almost the whole of the lateral surface of the mandibular ramus. Most of its fibres slope downwards and backwards at 45°. The posteriormost fibres arise from the deep surface of the arch and pass vertically downwards to be inserted into the upper part of the ramus; these fibres blend with the lower fibres of temporalis. The upper anterior part of the muscle is covered by an aponeurosis on which the parotid duct and the accessory parotid gland lie.

The muscle receives blood supply from branches of the facial artery, maxillary artery and superficial temporal artery, particularly its transverse facial artery. These vessels form an anastomotic network on the surface of and within the muscle.

Nerve supply. By the masseteric branch of the mandibular nerve, which passes through the mandibular notch to enter the deep surface of the gland.

Action. Masseter elevates and draws forwards the angle of the mandible when the jaws are approximated. The deep fibres assist temporalis in retracting the mandible.

The parotid gland is the largest of the major salivary glands, i.e. glands that drain saliva into the mouth through ducts. It is a mainly serous gland, with only a few scattered mucous acini. It is a large, irregular, lobulated gland which extends from the zygomatic arch to the upper part of the neck, where it overlaps the posterior belly of digastric and the anterior border of sternocleidomastoid (Fig. 6.16). Anteriorly the gland overlaps masseter and a small, usually detached accessory parotid lies above the parotid duct on the aponeurotic part of masseter. The gland extends below the external acoustic meatus posteriorly onto the mastoid process. In transverse section the gland is wedge-shaped, occupying the gap between the ramus of the mandible and the mastoid and styloid processes of the temporal bone, and reaching close to the lateral wall of the oropharynx; hence the need to look at the region of the fauces when examining a patient with a parotid mass.

The lateral (superficial) surface of the gland is covered by skin and superficial fascia. The investing layer of deep cervical fascia splits to envelope the gland and the inner leaf passes up to the base of the skull (see p. 330). The outer leaf extends superiorly as the parotidomasseteric fascia and reaches up to the zygomatic arch. On the gland, the fascia tends to be termed the parotid capsule and, more anteriorly, the masseteric fascia. Overlying the gland is a superficial muscular aponeurotic system (SMAS), which is continuous above with the temporoparietal fascia (see p. 356) and frontalis, below with platysma and over the gland with risorius. SMAS is adherent to the parotidomasseteric fascia in the pretragal area and becomes separate from it as the fascia enters the cheek where it overlies the parotid duct, facial nerve branches and buccal fat pad. The nerve branches penetrate the parotidomasseteric fascia as they proceed peripherally to inervate overlying facial muscles. The great auricular nerve supplies the fascia superficial and deep to the parotid gland, and transmits the pain caused by stretching of the fascial envelope when acute enlargement of the gland occurs as in mumps.

The anteromedial surface is grooved by the posterior border of the mandibular ramus, and is related to the masseter and medial pterygoid muscles which are attached to the ramus. The gland is also wrapped around the capsule of the temporomandibular joint. The anterior edge of this surface meets the lateral surface over, as well as below, the masseter forming the irregularly convex anterior border of the gland. The parotid duct and the facial nerve branches emerge from the anteromedial surface and run forwards deep to the anterior border. The terminal branches of the external carotid artery (superficial temporal and maxillary) leave this surface further back.

The posteromedial surface is in contact with the mastoid process with its attached sternocleidomastoid and posterior belly of digastric muscles. More medially, the styloid process and its attached muscles (stylohoid, stylopharyngeus and styloglossus) separate the gland from the carotid sheath and its contained internal jugular vein and internal carotid artery. The external carotid artery enters the gland through the lower part of this surface. The facial nerve trunk, or its temporofacial and cervicofacial divisions, enter the gland between the mastoid and styloid processes.

Within the gland the branches of the facial nerve run in different directions corresponding with their destinations, i.e. scalp, eyelids, mid-face, lower face and neck, and they do so in different (superficial to deep) planes. There is no specific, developmentally determined plane in which the facial nerve branches pass between superficial and deep lobes of the gland; the parotid is an integral gland, not divided into lobes. Within the gland the nerve branches communicate with each other, forming a plexiform arrangement that lies superficial to the retromandibular vein, which in turn is superficial to the external carotid artery. The retromandibular vein is formed within the parotid by the confluence of the superficial temporal and maxillary veins. The retromandibular vein emerges from the lower part (pole) of the gland and divides into an anterior branch which joins the facial vein and a posterior branch which joins the posterior auricular vein to form the external jugular vein; however, the division may occur within the gland and the two branches emerge from the lower pole. Lymph nodes of the preauricular (parotid) group lie on or deep to the fascial capsule of the parotid, as well as within the gland.

The parotid duct (of Stensen), about 5cm long, passes forwards across the masseter and turns around its anterior border to pass through the buccal fat pad and pierce the buccinator. It lies on the middle third of a line between the intertragic notch of the auricle and the midpoint of the philtrum (the vertical midline groove between the nasal septum and the upper lip) and is palpable on the clenched masseter muscle. The duct opens on the mucous membrane of the cheek opposite the second upper molar tooth (Fig. 6.17); it pierces the buccinator further back and runs forwards beneath the mucous membrane to its orifice. When intraoral pressure is raised this submucous part of the duct is compressed between the buccinator and the mucous membrane, preventing inflation of the gland.

An accessory parotid gland usually lies on the masseter between the duct and the zygomatic arch. Several small ducts open from it into the parotid duct. It and the duct lie on the aponeurotic part of the surface of the masseter muscle.

Blood supply

Branches from the external carotid artery supply the gland. Venous return is to the retromandibular vein.

Lymph drainage

Lymph drains to the preauricular (parotid) nodes and thence to nodes of the upper group of deep cervical nodes.

Nerve supply

Secretomotor fibres arise from cell bodies in the otic ganglion (see p. 22) and reach the gland by ‘hitch-hiking’ along the auriculotemporal nerve. As it passes backwards along the mandibular neck and ascends behind the temporomandibular joint, the auriculotemporal nerve is in contact with the anteromedial surface of the gland, which is penetrated by filaments from the nerve. The preganglionic fibres arise from cell bodies in the inferior salivary nucleus in the medulla, and travel by way of the glossopharyngeal nerve, its tympanic branch, the tympanic plexus and the lesser petrosal nerve to the otic ganglion. Sympathetic (vasoconstrictor) fibres reach the gland from the superior cervical ganglion by way of the plexus on the external carotid and middle meningeal arteries.

Development

A groove that appears in the ectoderm of the mouth pit (stomodeum, see p. 28) becomes converted into a tunnel, from the blind end of which cells proliferate to form the gland.

Surgical approach

The most common neoplasm of the parotid gland is a pleomorphic adenoma (mixed parotid tumour) which requires removal with a margin of normal parotid tissue, conserving the facial nerve and its branches. On account of the wide extent of the gland, it is approached through an S-shaped incision made from in front of the ear, backwards to the mastoid process and then downwards and forwards below the angle of the mandible. The gland is retracted forwards from the sternocleidomastoid to expose the posterior belly of digastric and stylohyoid and the cartilage of the external meatus. The facial nerve is approached along a plane in front of the anterior margin of the cartilage. The trunk emerges from the stylomastoid foramen, just deep to the junction of the cartilaginous and bony parts of the external meatus, about 1cm above and medial to the upper end of the posterior belly of digastric (Fig. 6.18). The cartilage in this region has a slight arrow-headed projection that points downwards to the emerging nerve trunk. The stylomastoid branch of the posterior auricular artery is superficial to the facial nerve and is a guide to its proximity. Once identified, the facial nerve is followed forwards into the gland and the required amount of parotid tissue removed with preservation of facial nerve branches. An alternative approach to facial nerve conservation is to first find a facial nerve branch as it leaves the gland and to follow this in a centripetal manner back to the trunk and other branches. The marginal mandibular branch may be identified as it lies superficial to the retromandibular vein or its anterior branch, aided by the colour contrast between the white nerve and the dark vein; occasionally this branch may pass behind the vein. Alternatively, the cervical branch may be followed up from its communication with the ascending branch of the transverse cervical nerve.