Orbital Anatomy Made Easy: Walls, Fissures and Muscles
Last updated: October 2026
The orbit is small but crowded. This page organises the bones, openings and muscles so that the applied anatomy (fractures, apex syndromes, blocks) makes sense.
Key numbers
- Volume about 30 mL; depth about 45 mm; the globe occupies only about a fifth of the orbit.
- The lateral walls are at about 90° to each other; the medial walls are nearly parallel; each orbital axis lies about 22.5° from the midline.
Walls: seven bones, four walls
| Wall | Bones | Applied point |
|---|---|---|
| Roof | Frontal bone; lesser wing of sphenoid | Fractures can cause CSF leak; frontal sinus mucocoele |
| Lateral | Zygomatic bone; greater wing of sphenoid | Thickest and strongest wall; protects only the back half of the globe |
| Floor | Maxilla (most); zygomatic; palatine (small posterior part) | Commonest blowout site; infraorbital nerve runs in it (cheek numbness) |
| Medial | Frontal process of maxilla; lacrimal bone; ethmoid (lamina papyracea); body of sphenoid | Thinnest wall; route for ethmoid sinusitis to cause orbital cellulitis |
Memory hook: the roof and lateral wall each have two bones, the floor three and the medial wall four: "2–2–3–4".
Openings
| Opening | Contents |
|---|---|
| Optic canal (lesser wing of sphenoid) | Optic nerve, ophthalmic artery, sympathetic fibres |
| Superior orbital fissure: outside the annulus | Lacrimal, frontal and trochlear nerves ("LFT"), superior ophthalmic vein |
| Superior orbital fissure: inside the annulus | Superior and inferior divisions of III, nasociliary nerve, VI ("NO" = Nasociliary and Oculomotor, plus abducens) |
| Inferior orbital fissure | Infraorbital and zygomatic nerves, infraorbital artery, branches of the inferior ophthalmic vein |
| Nasolacrimal canal | Nasolacrimal duct to the inferior meatus |
Extraocular muscles
| Muscle | Primary action | Secondary | Tertiary | Nerve |
|---|---|---|---|---|
| Medial rectus | Adduction | – | – | III (inferior division) |
| Lateral rectus | Abduction | – | – | VI |
| Superior rectus | Elevation | Intorsion | Adduction | III (superior division) |
| Inferior rectus | Depression | Extorsion | Adduction | III (inferior division) |
| Superior oblique | Intorsion | Depression | Abduction | IV |
| Inferior oblique | Extorsion | Elevation | Abduction | III (inferior division) |
- LR6 SO4, the rest 3. The recti arise from the annulus of Zinn; the superior oblique from the sphenoid body (through the trochlea); the inferior oblique from the orbital floor near the lacrimal fossa, the only muscle not arising from the apex.
- Spiral of Tillaux (insertion distance from the limbus): medial 5.5 mm, inferior 6.5 mm, lateral 6.9 mm, superior 7.7 mm.
- The superior oblique has the longest tendon; the inferior oblique has almost none.
- "SIN RAD": Superiors INtort; Recti (vertical) ADduct.
Vessels and nerves
- The ophthalmic artery (from the internal carotid) gives the central retinal artery, posterior ciliary arteries, muscular branches (anterior ciliary arteries), lacrimal, supraorbital and ethmoidal branches.
- The superior ophthalmic vein drains to the cavernous sinus; facial connections explain cavernous sinus thrombosis from facial infection.
- The ciliary ganglion lies between the optic nerve and lateral rectus; only parasympathetic fibres (from the nerve to the inferior oblique) synapse there.
Applied anatomy
- Superior orbital fissure syndrome: III, IV, V1 and VI palsies. Orbital apex syndrome: the same plus optic neuropathy. Cavernous sinus syndrome: may add V2 and Horner syndrome.
- Blowout fractures trap the inferior rectus (floor) or medial rectus (medial wall).
- Retrobulbar blocks are placed in the muscle cone; the optic nerve sheath risk explains brainstem anaesthesia.
Related: orbital fracture, orbital cellulitis, nerve palsies, eye embryology.
Key sources
- Snell RS, Lemp MA. Clinical Anatomy of the Eye, 2nd ed. Blackwell; 1998
- American Academy of Ophthalmology. BCSC Section 2 and Section 7 (current editions)
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