Muscles, Foramina & Nerves

The orbital muscles, bony passages, and cranial nerves are organized around the orbital apex. The four rectus muscles arise from the annulus of Zinn, while the optic canal and orbital fissures provide distinct neurovascular pathways.Optic canal: CN II + ophthalmic artery • Superior orbital fissure: CN III, IV, V1, VI + superior ophthalmic vein

Reference Images

Axial CT extraocular muscle anatomy — Extraocular muscles on axial CT

Annulus of Zinn and orbital apex — Annulus of Zinn

Contents of orbital and facial foramina — Contents of foramina

Orbital apex and foraminal pathways — Orbital apex pathways

Supraorbital foramen and nerve — Supraorbital foramen

Sensory nerve branches around the orbit — Orbital sensory branches

Optic nerve and sheath complex — Optic nerve/sheath complex

Segments of the optic nerve — Optic nerve segments

Extraocular Muscles

Muscle	Origin / course	Main action	Other actions	Nerve
Medial rectus	Annulus → medial sclera	Adduction	—	CN III
Lateral rectus	Annulus → lateral sclera	Abduction	—	CN VI
Superior rectus	Annulus → superior sclera	Elevation	Adduction, intorsion	CN III
Inferior rectus	Annulus → inferior sclera	Depression	Adduction, extorsion	CN III
Superior oblique	Sphenoid → trochlea → superolateral sclera	Intorsion	Depression, abduction	CN IV
Inferior oblique	Anterior orbital floor → inferolateral sclera	Extorsion	Elevation, abduction	CN III
Levator	Lesser sphenoid wing → upper lid	Lid elevation	—	CN III

Key relationships

The recti form the muscle cone and define the intraconal space. The insertion order from closest to farthest from the limbus is medial rectus 5.5 mm → inferior rectus 6.5 mm → lateral rectus 6.9 mm → superior rectus 7.7 mm (spiral of Tillaux).

Actions & Testing

Mnemonic: LR6 SO4, all the rest III.
Superior rectus: abduct, then look up.
Inferior oblique: adduct, then look up.
Inferior rectus: abduct, then look down.
Superior oblique: adduct, then look down.
Superior oblique travels through the trochlea.
Inferior oblique uniquely originates in the anterior orbit.

Foramina & Fissures

Opening	Connection	Principal contents
Optic canal	Orbital apex	CN II, ophthalmic artery
Superior orbital fissure	Cavernous sinus region → orbit	CN III, IV, V1, VI, superior ophthalmic vein
Inferior orbital fissure	Orbit ↔ pterygopalatine/infratemporal regions	V2 branches, inferior ophthalmic vein
Foramen rotundum	Middle cranial fossa → PPF	V2
Foramen ovale	Middle cranial fossa → infratemporal fossa	V3
Supraorbital notch/foramen	Superior orbital rim	Supraorbital nerve, artery, vein
Infraorbital foramen	Anterior maxilla	Infraorbital nerve and vessels

Bony Orbit

Frontal: roof and superior rim.
Sphenoid: posterior orbit and apex.
Ethmoid: medial wall.
Lacrimal: anterior medial wall.
Maxilla: floor and inferomedial rim.
Zygomatic: lateral wall and rim.
Palatine: small posterior floor contribution.

Cranial Nerves

CN II: vision; passes with the ophthalmic artery through the optic canal.
CN III: SR, MR, IR, IO, LPS and parasympathetic pupillary fibers.
CN IV: superior oblique.
V1: orbit, cornea, conjunctiva, upper lid, forehead, and dorsum of nose.
V2: enters through inferior orbital fissure and continues as the infraorbital nerve.
CN VI: lateral rectus.

Optic Nerve & Sheath

The optic nerve is a CNS tract with oligodendrocytes.
Its dural sheath is continuous with intracranial dura and includes all three meningeal layers.
The perioptic subarachnoid space contains CSF and communicates with the suprasellar cistern.
Segments: intraorbital, canalicular, and cisternal.
The intraorbital segment has an S-shaped contour allowing globe movement.
Central retinal vessels pierce the dura in the mid-orbit and then travel within the nerve.

Palsy Patterns

CN III: ptosis, impaired adduction/elevation/depression, possible mydriasis; eye typically “down and out.”
CN IV: impaired depression in adduction; vertical/torsional diplopia worse in downgaze.
CN VI: impaired abduction and horizontal diplopia worse toward the affected side.

Imaging Localization

Pattern	Suggested localization
Visual loss without ocular-motor palsy	Optic nerve or optic canal
Visual loss plus III, IV, VI palsies and V1 sensory loss	Orbital apex / superior orbital fissure
III, IV, VI palsies plus V1/V2 sensory loss	Cavernous sinus or posterior orbital/SOF region
Lower-lid, cheek, lateral-nose, upper-lip, or upper-teeth numbness	Infraorbital nerve/V2 pathway
Forehead or scalp numbness	Supraorbital nerve / frontal V1 branch
Orbital-floor fracture with cheek paresthesia	Infraorbital canal and nerve

Muscle assessment

Report the involved muscles, tendon sparing or involvement, symmetry, adjacent fat inflammation, lacrimal-gland involvement, apical crowding, and optic-nerve compression.