2.1 Orbit, Eyelids, Lacrimal System, Conjunctiva & Extraocular Muscles

The bony orbit

The orbit is a four-walled pyramid formed by seven bones: frontal, zygomatic, maxillary, sphenoid (greater and lesser wings), ethmoid, lacrimal and palatine. Total orbital volume is about 30 mL; the globe occupies roughly 6.5 mL, leaving fat, muscle, vessels and nerves to fill the rest. That ratio explains why a few millilitres of retrobulbar blood produce dramatic proptosis and a pressure emergency.

Three openings matter for technologist work. The optic canal transmits the optic nerve and ophthalmic artery. The superior orbital fissure transmits cranial nerves III, IV, V1 and VI plus the superior ophthalmic vein — which is why a superior orbital fissure syndrome produces a total ophthalmoplegia. The inferior orbital fissure transmits V2 and the infraorbital vessels; hypoaesthesia of the cheek after blunt trauma points there.

Eyelids

From front to back the lid is skin, orbicularis oculi, orbital septum, tarsal plate with the levator aponeurosis and Müller's muscle, and conjunctiva. Two retractors open the upper lid:

• Levator palpebrae superioris — cranial nerve III, provides most of the 15 mm excursion.
• Müller's muscle — sympathetic innervation, contributes 2 mm.

That 2 mm is the whole explanation for Horner syndrome: sympathetic denervation produces a mild ptosis with miosis and anhidrosis, never the complete lid droop of a third-nerve palsy. Orbicularis oculi closes the lid and is innervated by cranial nerve VII; its failure causes lagophthalmos and exposure keratopathy.

The tarsal plates contain the meibomian glands, which secrete the lipid layer of the tear film. Glands of Zeis (sebaceous) and Moll (apocrine) open at the lash line. A blocked meibomian gland with granulomatous inflammation is a chalazion; an acute infected gland of Zeis or Moll is an external hordeolum.

The lacrimal system

Production. The main lacrimal gland sits in the superolateral orbit, divided by the levator aponeurosis into orbital and palpebral lobes, and provides reflex aqueous tearing via parasympathetic fibres travelling with cranial nerve VII. Accessory glands of Krause and Wolfring supply basal aqueous secretion. The tear film has three contributions: an outer lipid layer from the meibomian glands that retards evaporation, a middle aqueous layer, and an inner mucin layer from conjunctival goblet cells that lets the aqueous wet the hydrophobic epithelium.

Drainage. Tears move by the lacrimal pump into:

1. Puncta — upper and lower, at the medial lid margin
2. Canaliculi — 2 mm vertical, then 8 mm horizontal
3. Common canaliculus and the valve of Rosenmüller
4. Lacrimal sac
5. Nasolacrimal duct, ending at the inferior meatus under the inferior turbinate (valve of Hasner)

Epiphora with a sticky, distended medial canthus suggests sac obstruction; a congenital membranous obstruction at the valve of Hasner is the usual cause of infant tearing.

Conjunctiva

Three zones: palpebral (lining the lids, firmly adherent), forniceal (loose, redundant, allowing globe rotation) and bulbar (covering the sclera to the limbus, over Tenon's capsule). The semilunar fold and the caruncle sit medially. Goblet cells are densest in the fornices. Because the palpebral conjunctiva is adherent and the bulbar is loose, subconjunctival haemorrhage spreads over the globe and stops sharply at the limbus.

Extraocular muscles

Six muscles move each globe. Four recti arise from the annulus of Zinn and insert anterior to the equator; the superior oblique arises from the orbital apex and passes through the trochlea; the inferior oblique arises from the maxilla near the lacrimal fossa.

The rectus insertions form the spiral of Tillaux, measured from the limbus: medial 5.5 mm, inferior 6.5 mm, lateral 6.9 mm, superior 7.7 mm. Memorise the order — it is a standing exam item and it underpins muscle surgery landmarks.

The obliques act as elevator and depressor in adduction and the vertical recti act in abduction, which is the basis for testing the six cardinal positions and for the three-step test used to isolate a cyclovertical palsy.