Human Anatomy
Orbit
Orbit anatomy for MBBS: bony boundaries and openings, extraocular muscles and their nerve supply, ocular nerve palsies and the ciliary ganglion, mapped to NMC codes AN31.1 to AN31.5.
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Orbit
Orbit anatomy for MBBS: bony boundaries and openings, extraocular muscles and their nerve supply, ocular nerve palsies and the ciliary ganglion, mapped to NMC codes AN31.1 to AN31.5.
The orbit chapter covers the bony cavity of the eye, its openings, the extraocular muscles and their nerve supply, and the neurovascular contents at the apex. It is the anatomical basis for the ocular nerve palsies and vascular emergencies that recur throughout medicine and ophthalmology.
High-yield: Orbit
- The orbit is a pyramidal cavity with the base facing forward and the apex at the optic canal.
- The optic canal transmits the optic nerve and the ophthalmic artery.
- The superior orbital fissure transmits the oculomotor, trochlear, abducent and ophthalmic nerves, along with the ophthalmic veins.
- There are seven muscles in the orbit: four recti, two obliques and levator palpebrae superioris.
- All the extraocular muscles are supplied by the oculomotor nerve except the superior oblique, which is supplied by the trochlear nerve, and the lateral rectus, by the abducent nerve.
- The rule is remembered as LR6 SO4, meaning lateral rectus by the sixth nerve and superior oblique by the fourth.
- The superior oblique turns the eye down and out, so trochlear nerve palsy causes double vision on looking down, as when descending stairs.
- Abducent nerve palsy leaves the eye adducted at rest because the medial rectus is unopposed.
- A complete oculomotor nerve palsy gives ptosis, a down and out eye and a fixed dilated pupil.
- The levator palpebrae superioris has a smooth muscle part supplied by sympathetic fibres, so loss of these fibres causes the partial ptosis of Horner's syndrome.
- The ciliary ganglion lies near the apex of the orbit and relays parasympathetic fibres to the sphincter pupillae and ciliary muscle.
- The retina is supplied by the central artery of the retina, an end artery, so its occlusion causes sudden painless blindness.
- The four recti arise from a common tendinous ring around the optic canal.
- The orbital fat cushions the eyeball and its loss produces the sunken eye of enophthalmos.
Extraocular muscle nerve supply
- **Oculomotor nerve:** Superior, inferior and medial recti, inferior oblique and levator palpebrae superioris.
- **Trochlear nerve:** Superior oblique only. Palsy gives vertical diplopia on looking down.
- **Abducent nerve:** Lateral rectus only. Palsy leaves the eye adducted with medial squint.
- **Ciliary ganglion:** Parasympathetic relay to sphincter pupillae and ciliary muscle.
NMC competencies in this chapter
- **AN31.1:** Bony boundaries and openings of the orbit
- **AN31.2:** Extraocular muscles: attachments, actions and nerve supply
- **AN31.3:** Nerves and vessels of the orbit and the ciliary ganglion
- **AN31.4:** Effects of palsy of the ocular motor nerves
- **AN31.5:** Applied anatomy of the orbit
Frequently Asked Questions
How do you remember the nerve supply of the eye muscles?
Use LR6 SO4: the lateral rectus is supplied by the sixth nerve and the superior oblique by the fourth nerve, and the oculomotor nerve supplies all the rest along with levator palpebrae superioris.
Why does an oculomotor nerve palsy give a down and out eye?
With the oculomotor-supplied muscles paralysed, the unopposed lateral rectus and superior oblique pull the eye down and outward, and there is ptosis with a fixed dilated pupil.
Why is central retinal artery occlusion an emergency?
The central artery of the retina is an end artery with no anastomosis, so its blockage cuts off the inner retina and causes sudden painless loss of vision in that eye.
How does this chapter help in NEET-PG?
Ocular nerve palsies, the ciliary ganglion and retinal blood supply are frequently tested in ophthalmology and neurology, so orbital anatomy is directly high-yield for PG.
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