Parotid Region & Mastication Muscles

Parotid Region

• The parotid region is below and in front of the ear, and below the zygomatic arch.
• Key features of this region are the parotid gland and the masseter muscle.
• Other muscles of mastication include the temporalis muscle, and the medial and lateral pterygoid muscles.

Masseter

• Arises from the zygomatic arch with three heads fused on the arch.
• Inserts on the mandibular ramus, with three sets of insertions that fuse anteriorly but diverge posteriorly to allow passage of the masseteric nerve and artery.
• The superficial part arises from the anterior two-thirds of the lower border of the zygomatic arch, with fibers sloping down at 45 degrees, inserting into the angle of the mandible and along the lower part of the ramus.
• The upper part is covered in aponeurotic fibers, while the lower half is fleshy.
• The intermediate part arises from the middle third of the arch and the deep part from the deep surface of the arch.
• These parts insert vertically into the ramus of the mandible, fusing anteriorly but remaining separate posteriorly.
• The masseteric nerve runs between the deep and intermediate parts.
• A branch from the superficial temporal or transverse facial artery runs between the superficial and intermediate parts.
• The masseteric branch from the anterior division of the mandibular nerve provides nerve supply.
• Action: closes the jaws by elevating and drawing forward the angle of the mandible.

Parotid Gland

• Predominantly a serous salivary gland, with few scattered mucous acini.
• Has an irregular shape, filling the gap between the mastoid process, ramus of the mandible, and styloid process.
• It has upper and lower poles and lateral, anterior, and deep surfaces.
• Surrounded by a tough capsule, the parotid sheath, derived from the investing layer of deep cervical fascia.
• Virus infections of the gland, like mumps, is painful because the gland swells within the tight fibrous envelope.
• The upper pole adheres to the cartilage of the external acoustic meatus and lies adjacent to the capsule of the temporomandibular joint.
• The lower pole is rounded, lying below and behind the angle of the mandible, indented by it and sternocleidomastoid, and overlapping the posterior belly of digastric.
• The lateral surface is subcutaneous and almost flat.
• The anterior surface clasps the ramus of the mandible with masseter on its outer surface and the medial pterygoid on its inner surface inferiorly.
• The stylomandibular ligament separates the anterior surface from the medial pterygoid and the submandibular gland.
• The outer edge of the anterior surface meets the lateral surface over masseter to form the convex anterior border, where the parotid duct and the five branches of the facial nerve emerge.
• Terminal branches of the external carotid artery leave the gland from the deeper part of the anterior surface.
• The deep surface is indented by the mastoid process and its attached muscles, and lies against the styloid process with its attached muscles and ligaments.
• The external carotid artery enters the gland through the lower part of the deep surface, which joins the deep edge of the anterior surface.
• The styloid process separates the gland from the internal jugular vein and the internal carotid artery.
• The temporozygomatic and cervicofacial branches of the facial nerve enter the gland between the styloid and mastoid processes.
• A small arrow-like projection of the meatal cartilage points downwards towards the facial nerve.
• Embedded within the gland are the facial nerve, retromandibular vein, and external carotid artery, from superficial to deep.
• Branches of the facial nerve emerge from behind the anterior border.
• Immediately deep to the plane of the nerve branches is the retromandibular vein, which can be a guide to the position of the nerves.
• The external carotid artery and its two terminal branches are the deepest structures within the gland.
• Lymph nodes of the preauricular group may be within the gland substance or just inside the capsule, and the gland is penetrated by filaments of the auriculotemporal nerve.
• The parotid duct (of Stensen), 5 cm long, passes forwards across the masseter and turns around its anterior border to pierce the buccinator.
• The parotid duct lies in the line between the intertragic notch of the auricle and the midpoint of the philtrum.
• The parotid duct opens on the mucous membrane of the cheek opposite the second upper molar tooth; it pierces the buccinator further back and runs forwards beneath the mucous membrane to its orifice.
• An accessory parotid gland usually lies on the masseter between the duct and the zygomatic arch, with several ducts opening into the parotid duct.

Parotid Gland: Blood Supply, Lymph Drainage, Nerve Supply & Structure

• Blood supply is from branches of the external carotid artery and venous return is to the retromandibular vein.
• Lymph drains to the nodes within the parotid sheath and then to nodes of the upper group of deep cervical nodes.
• Secretomotor nerve fibers arise from cell bodies in the otic ganglion.
• Preganglionic fibers arise from the inferior salivary nucleus in the medulla, traveling via the glossopharyngeal nerve, its tympanic branch, the tympanic plexus, and the lesser petrosal nerve to the otic ganglion.
• Sympathetic (vasoconstrictor) fibers reach the gland from the superior cervical ganglion via the plexus on the external carotid and middle meningeal arteries.
• Sensory fibers are from the auriculotemporal nerve (gland itself), great auricular nerve (parotid fascia).
• Histological features: predominantly serous acini, many ducts, and fat cells scattered between the acini and ducts.
• Development: A groove in the cheek (ectoderm) becomes a tunnel, from which cells proliferate to form the gland.

Surgical Approach: Parotidectomy

• Involves dissection of the facial nerve.
• An S-shaped incision is made from in front of the ear, backwards to the mastoid process, and then downwards and forwards below the angle of the mandible.
• The skin incision includes platysma.
• The gland is retracted forwards from sternocleidomastoid to expose the posterior belly of digastric and stylohyoid and the cartilage of the external meatus.
• The trunk of the facial nerve is approached in front of the anterior margin of the cartilage, above stylohyoid and digastric.
• The stylomastoid branch of the posterior auricular artery is a guide to the proximity of the facial nerve.
• The cartilage in this region has a slight arrow-headed projection that points towards the nerve, which can be followed forward along its various branches.
• The parotid duct is divided at the anterior border of the masseter.
• Total parotidectomy involves further dissection with ligation of the external carotid, superficial temporal, and maxillary arteries and adjacent veins.
• The styloid process may have to be fractured and the stylomandibular ligament divided.

Overview of the parotid region's anatomy, including the parotid gland and masseter muscle. Details the masseter's origin from the zygomatic arch and its insertion on the mandibular ramus. Explains the different parts and fiber arrangements of the masseter muscle.