Submandibular — MRCS Part B Anatomy

The face and the submandibular region dissected: the muscles of expression, the gland with its duct, the vessels that cross it, and the two nerves at risk when it is removed.

The gland

A mixed but mainly serous salivary gland lying in the digastric triangle, in the submandibular fossa on the inner surface of the mandible below the mylohyoid line. Most of it is superficial to mylohyoid, under the skin, platysma and the investing layer of deep cervical fascia, and is what is felt below the angle of the jaw; a small deep part hooks round the free posterior border of mylohyoid into the floor of the mouth. The duct, about 5 cm long, leaves the deep part and runs forward on hyoglossus, between the lingual nerve above and the hypoglossal nerve below, to open at the sublingual papilla beside the frenulum of the tongue. Because its saliva is thicker and its duct runs uphill, this is the gland that forms stones: four out of five salivary calculi are here.

Blood supply and innervation

The facial artery reaches it from the external carotid, grooves its deep surface or passes through it, and emerges over the lower border of the mandible at the front edge of masseter, where its pulse can be felt; the facial vein runs across the surface of the gland. Lymph drains to the submandibular nodes that lie on and in the gland, and from there to the deep cervical chain. Its secretomotor supply is parasympathetic from the facial nerve: fibres leave in the chorda tympani, join the lingual nerve, and synapse in the submandibular ganglion, which hangs from the lingual nerve above the gland; sympathetic fibres reach it along the facial artery.

Relations

The lingual nerve above the duct, curling under it from lateral to medial; the hypoglossal nerve below, on hyoglossus, with its accompanying vein; mylohyoid on its lateral side with the nerve to mylohyoid; hyoglossus deep to it; and the anterior and posterior bellies of digastric marking its lower boundary. The marginal mandibular branch of the facial nerve runs across it just under platysma, below the mandible.

The fascia

The investing layer of deep cervical fascia splits to enclose the gland in a capsule, attaching to the mandible above and the hyoid below. Because the gland is inside its own fascial compartment, an abscess in it is under tension, and infection from a lower molar spreading into the submandibular and sublingual spaces on both sides is Ludwig's angina, which pushes the tongue up and back and threatens the airway.

Removing the gland

The incision is placed two finger-breadths below the mandible to stay under the marginal mandibular nerve, which is then lifted with the platysma and the facial vein. Damage to it drops the corner of the mouth: the lower lip cannot be drawn down and out, which shows when the patient smiles or whistles. The lingual nerve is at risk when the duct is divided, giving numbness of the side of the tongue and loss of taste, and the hypoglossal nerve when the deep surface is freed, giving deviation of the tongue to that side.

The face

The muscles of expression named on the specimen — frontalis, orbicularis oculi, the levators of the upper lip, zygomaticus, orbicularis oris, buccinator, the depressors of the lower lip — are all facial nerve muscles, arising from bone and inserting into skin, and lying in the superficial fascia with the nerve's branches between them and masseter. The angular vein at the medial canthus connects the facial vein with the ophthalmic veins and the cavernous sinus, which is the route for infection from the "danger area" of the face.