Describe the parotid gland under bed, structures within it, duct and nerve supply. Add a note on its applied anatomy.

General Features

• Weighs about 25 g; roughly wedge-shaped with an apex below and a base above, and three surfaces
• Enclosed in a tough capsule from the investing layer of the deep cervical fascia, split to enclose it

The Parotid Bed

• Anteriorly — ramus of the mandible, masseter, medial pterygoid
• Posteriorly — mastoid process, sternocleidomastoid, posterior belly of digastric, styloid process with its muscles
• Medially — styloid apparatus, and beyond it the internal carotid artery, internal jugular vein and the last four cranial nerves
• Superiorly — external acoustic meatus and the temporomandibular joint

Structures Within the Gland

• Mnemonic — "Nerve, Vein, Artery" from outside in; the arterial structure is deepest, the reverse of the usual arrangement
• The facial nerve divides the gland into a superficial and a deep lobe, a surgical rather than an anatomical division
• The five terminal branches — temporal, zygomatic, buccal, marginal mandibular, cervical; mnemonic "Ten Zebras Bit My Cat"

The Parotid Duct

• Stensen duct — about 5 cm long
• Emerges from the anterior border, runs across masseter, turns medially at its anterior border, pierces buccinator
• Opens into the vestibule of the mouth opposite the crown of the upper second molar tooth
• Surface marking — the middle third of a line from the intertragic notch to the midpoint between the ala of the nose and the upper lip
• The oblique passage through buccinator acts as a valve, preventing inflation of the duct during blowing

Nerve Supply

• Sympathetic — from the superior cervical ganglion, along the external carotid plexus; vasomotor
• Sensory to the capsule — great auricular nerve (C2, C3) and the auriculotemporal nerve

Blood Supply, Lymphatics and Comparison

• Arteries — branches of the external carotid within the gland; the transverse facial artery supplies the duct
• Veins — the retromandibular vein, formed within the gland by the union of the superficial temporal and maxillary veins; it divides into anterior and posterior branches at the lower pole
• Lymph — to the parotid nodes (some lying within the gland), then to the deep cervical chain. This is why the parotid may harbour metastases from the scalp and eyelid

• The rule of proportion — the larger the gland, the more likely a tumour is benign; about 80% of parotid tumours are benign, but the majority of sublingual and minor gland tumours are malignant

Applied Aspects

• Mumps — viral parotitis; painful because the tough fascial capsule will not stretch. Pain is worse on eating, and complications include orchitis, oophoritis, pancreatitis and meningitis
• Pain is referred to the ear, since the auriculotemporal nerve supplies both the gland capsule and the external auditory meatus
• Parotid abscess must be drained by a horizontal (transverse) incision, parallel to the branches of the facial nerve, to avoid dividing them
• Pleomorphic adenoma is the commonest tumour, usually in the superficial lobe; treated by superficial parotidectomy, never by enucleation, which causes recurrence
• Facial nerve palsy with a parotid mass suggests malignancy — a benign tumour displaces the nerve but does not paralyse it
• The deep lobe can be palpated intra-orally, bulging into the lateral pharyngeal wall
• Parotid fistula may follow injury to the duct where it crosses masseter, the most superficial part of its course
• Sjogren syndrome causes bilateral painless parotid enlargement with dry eyes and dry mouth, and carries a raised risk of lymphoma
• Xerostomia after radiotherapy to the head and neck reflects the sensitivity of serous acini, and causes rampant dental caries
• The facial nerve is identified at the tragal pointer and the posterior belly of digastric during parotidectomy — two reliable landmarks used before the nerve is traced forward
• Bilateral parotid enlargement suggests mumps, sarcoidosis, Sjogren syndrome, HIV or alcoholism rather than a tumour
• Sialogogues and massage help clear a duct obstructed by a mucous plug before any surgical intervention is considered
• Warthin tumour is the second commonest parotid tumour, benign, often bilateral, and strongly associated with smoking
• The parotid is the only salivary gland with lymph nodes inside it, because it becomes encapsulated late in development, after the lymphatic system has formed
• Recurrent parotitis of childhood presents with repeated painful swelling and usually resolves at puberty
• The great auricular nerve is sacrificed or preserved in parotidectomy; its loss numbs the lower ear lobe, which patients notice when wearing earrings
• A parotid swelling lifts the ear lobe outward, which distinguishes it from an upper cervical node
• Facial nerve monitoring is now routine during parotid surgery