Orbit and Eye

A pyramidal bony cavity, apex pointing posteromedially, base (orbital opening) facing anterolaterally, formed by contributions from seven bones: frontal, zygomatic, maxilla, ethmoid, lacrimal, palatine, sphenoid.

Roof:

• Formed mainly by the orbital plate of the frontal bone, with a small posterior contribution from the lesser wing of sphenoid; separates the orbit from the anterior cranial fossa, thin, and a site of potential intracranial spread of orbital infection/tumour, or conversely, of anterior fossa fracture extending into the orbit.

Floor:

• Formed mainly by the orbital plate of the maxilla, with contributions from the zygomatic and palatine bones; separates the orbit from the maxillary sinus below, notably thin, making it the classic site of orbital blowout fracture (see Applied Anatomy).

Medial wall:

• Formed by the ethmoid bone (orbital/lamina papyracea), lacrimal bone, frontal process of the maxilla, and a small part of the sphenoid body, the thinnest wall of the orbit, separating it from the ethmoidal air cells, making it particularly vulnerable to both fracture and direct spread of ethmoidal sinus infection into the orbit.

Lateral wall:

• Formed by the zygomatic bone and the greater wing of sphenoid: the thickest and strongest orbital wall, providing the most protection against direct lateral trauma.

Apex:

• The posterior, narrowing point of the orbit, where the optic canal and the medial end of the superior orbital fissure are located, the site of convergence for the major neurovascular structures entering/exiting the orbit.

Orbital openings:

• Optic canal (optic nerve, ophthalmic artery), superior orbital fissure (CN III, IV, V1, VI, superior ophthalmic vein), inferior orbital fissure (CN V2/infraorbital nerve, infraorbital vessels, communicates with the pterygopalatine fossa), and smaller foramina for the supraorbital, infraorbital, and nasolacrimal structures.

Superior rectus:

• Origin: Common tendinous ring (annulus of Zinn), at the orbital apex.
• Insertion: Superior sclera, anterior to the equator of the eyeball.
• Innervation: Oculomotor nerve (CN III), superior division.
• Primary action: Elevation (with secondary actions of intorsion and adduction, given the muscle's oblique orientation relative to the eye's primary position).

Inferior rectus:

• Origin: Common tendinous ring.
• Insertion: Inferior sclera, anterior to the equator.
• Innervation: Oculomotor nerve (CN III), inferior division.
• Primary action: Depression (with secondary extorsion and adduction).

Medial rectus:

• Origin: Common tendinous ring.
• Insertion: Medial sclera, anterior to the equator.
• Innervation: Oculomotor nerve (CN III), inferior division.
• Primary action: Adduction (pure, no significant secondary action, given its direct alignment with the eye's primary position).

Lateral rectus:

• Origin: Common tendinous ring.
• Insertion: Lateral sclera, anterior to the equator.
• Innervation: Abducens nerve (CN VI).
• Primary action: Abduction (pure, no significant secondary action).

Superior oblique:

• Origin: Body of the sphenoid, above the common tendinous ring.
• Insertion: Passes through the fibrocartilaginous trochlea (a pulley-like structure on the anteromedial orbital roof), then angles posterolaterally to insert on the superior sclera, posterior to the equator.
• Innervation: Trochlear nerve (CN IV).
• Primary action: Intorsion (with secondary depression and abduction, most effective when the eye is already adducted, given the muscle's altered mechanical line of pull after passing through the trochlea).

Inferior oblique:

• Origin: Floor of the orbit, near the lacrimal fossa (anteriorly), the only extraocular muscle not arising from the common tendinous ring/orbital apex.
• Insertion: Posterolateral sclera, posterior to the equator.
• Innervation: Oculomotor nerve (CN III), inferior division.
• Primary action: Extorsion (with secondary elevation and abduction, most effective when the eye is already adducted).

Levator palpebrae superioris:

• Origin: Body of the sphenoid, above the common tendinous ring, superior to superior rectus's origin.
• Insertion: Upper eyelid tarsal plate and skin (with a smooth muscle component, the superior tarsal/Müller's muscle, receiving separate sympathetic innervation, relevant to Horner syndrome).
• Innervation: Oculomotor nerve (CN III), superior division (for the main skeletal muscle component); sympathetic fibres (for the superior tarsal muscle component).
• Action: Elevates the upper eyelid.

Mnemonic for oblique muscle actions: "obliques go the opposite way to their name", superior oblique depresses (despite "superior"), inferior oblique elevates (despite "inferior"), the naming reflects their position/origin, not their primary action.

Mnemonic for innervation: "LR6, SO4, all the rest are CN III", Lateral Rectus is CN VI, Superior Oblique is CN IV, everything else (SR, IR, MR, IO, LPS) is CN III.

Oculomotor nerve (CN III):

• Motor to superior, inferior, medial recti, inferior oblique, and levator palpebrae superioris; parasympathetic to the sphincter pupillae and ciliary muscle via the ciliary ganglion.

Trochlear nerve (CN IV):

• Motor to superior oblique only.

Abducens nerve (CN VI):

• Motor to lateral rectus only.

Course:

• Extends from the posterior eyeball (optic disc) through the orbit (within a dural/arachnoid/pial sheath, continuous with the intracranial meninges, a key fact explaining why raised intracranial CSF pressure can be transmitted along the nerve, causing papilloedema, visible on fundoscopy as optic disc swelling), through the optic canal, to the optic chiasm.

Optic canal:

• Located in the lesser wing of sphenoid, at the orbital apex, transmitting the optic nerve and the ophthalmic artery together.

Relations:

• Surrounded by the four rectus muscles' common tendinous ring origin at the orbital apex; closely related to the ophthalmic artery throughout the canal and proximal orbital course.

• Enters the orbit via the superior orbital fissure, dividing into three branches before entry (or just within the orbit):

Frontal nerve:

• The largest branch, running along the orbital roof, dividing into supraorbital and supratrochlear nerves, supplying the forehead/scalp and upper eyelid.

Lacrimal nerve:

• Runs along the lateral orbital wall to the lacrimal gland, providing sensory innervation to the gland and a small area of skin; also carries the postganglionic parasympathetic secretomotor fibres to the lacrimal gland via a communicating branch from the zygomatic nerve (itself carrying fibres originally from the pterygopalatine ganglion).

Nasociliary nerve:

• Crosses the orbit from lateral to medial, giving off the long ciliary nerves (sensory to the cornea and sympathetic fibres to the dilator pupillae, bypassing the ciliary ganglion), a sensory root to the ciliary ganglion, the posterior and anterior ethmoidal nerves (to the ethmoidal sinuses and nasal cavity), and terminating as the infratrochlear nerve (skin near the medial canthus).

Roots:

• Parasympathetic (motor) root: From CN III (specifically the nerve to inferior oblique), carrying preganglionic fibres that synapse here.
• Sympathetic root: Postganglionic fibres from the internal carotid plexus (already synapsed in the superior cervical ganglion in the neck), passing through without synapsing.
• Sensory root: From the nasociliary nerve (V1), also passing through without synapsing.

Connections:

• Located in the posterior orbit, lateral to the optic nerve: only the parasympathetic fibres actually synapse within the ganglion; the sympathetic and sensory fibres pass through it structurally but functionally bypass it (a common pattern for the parasympathetic ganglia of the head).

Short ciliary nerves:

• The post-ganglionic output of the ciliary ganglion, carrying the now-postganglionic parasympathetic fibres (to sphincter pupillae and ciliary muscle) along with the passenger sympathetic and sensory fibres, to the eyeball.

Ophthalmic artery:

• The first branch of the internal carotid artery after it emerges from the cavernous sinus, entering the orbit via the optic canal (with the optic nerve), giving rise to the central retinal artery and multiple ciliary arteries, along with branches supplying the extraocular muscles, eyelids, and (via the ethmoidal arteries) part of the nasal cavity, the orbit's dominant arterial supply.

Central retinal artery:

• A branch of the ophthalmic artery, piercing the optic nerve sheath and running within the nerve's substance to reach the retina, where it functions as an end-artery (essentially no significant collateral circulation), supplying the inner retinal layers.

Ciliary arteries:

• Multiple branches (short and long posterior ciliary arteries, anterior ciliary arteries) supplying the choroid, ciliary body, and iris, and (importantly) contributing collateral supply to the outer retinal layers (distinct from the central retinal artery's inner-layer territory) via the choroidal circulation.

Superior ophthalmic vein:

• The larger of the two ophthalmic veins, draining most of the orbit, communicating anteriorly with the facial vein (via the angular vein) and posteriorly draining into the cavernous sinus: a valveless connection of major clinical significance (see Applied Anatomy).

Inferior ophthalmic vein:

• Drains the inferior orbit, also communicating with the pterygoid venous plexus and the cavernous sinus (sometimes joining the superior ophthalmic vein before doing so, sometimes draining separately).

Cavernous sinus connections:

• Both ophthalmic veins' drainage into the cavernous sinus, combined with the facial vein's valveless connection via the angular/superior ophthalmic vein pathway, creates a direct route for infection to spread from the face/orbit into this critical intracranial venous structure.

Lacrimal gland:

• Located in the lacrimal fossa, a shallow depression in the anterolateral orbital roof (frontal bone), divided by the levator palpebrae superioris tendon into a larger orbital part and a smaller palpebral part; produces the aqueous component of tears, secreted via multiple small ducts opening into the superior conjunctival fornix.

Lacrimal ducts:

• The small excretory ducts of the lacrimal gland opening into the conjunctival sac, distinct from the drainage pathway.

Lacrimal sac:

• Located in the lacrimal fossa of the lacrimal bone (a different fossa from the gland's location, a common point of confusion, since both share the name "lacrimal fossa" but refer to entirely different locations/structures), at the medial canthus; receives tears via the small superior and inferior lacrimal canaliculi (which drain from the puncta on the eyelid margins), and drains inferiorly into the nasolacrimal duct.

Nasolacrimal duct:

• Continues from the lacrimal sac, descending through a bony canal to open into the inferior meatus of the nasal cavity, beneath the inferior concha, explaining why crying causes a "runny nose" (tears draining into the nasal cavity via this pathway), and why nasal congestion can sometimes cause secondary tear drainage problems (epiphora) if the duct's nasal opening becomes obstructed.

Orbital blowout fracture:

• A sudden, direct blunt force to the orbit (e.g., a fist or ball impact) transmits pressure through the relatively incompressible orbital contents, causing the thin orbital floor (most commonly) to fracture and "blow out" into the underlying maxillary sinus, can trap and entrap orbital contents (classically the inferior rectus muscle or surrounding fat) within the fracture line, causing restricted upward gaze (a positive "forced duction test") and diplopia on attempted vertical eye movement, along with infraorbital nerve numbness (given the infraorbital nerve's course through the orbital floor).

Ocular motor palsies:

• Isolated CN III, IV, or VI palsies produce the characteristic diplopia and eye position patterns described in Section 6, this section's detailed extraocular muscle action table (particularly the oblique muscles' counterintuitive naming vs action) is essential for correctly interpreting the specific pattern of diplopia/misalignment seen in each palsy.

Cavernous sinus spread:

• Given the orbit's rich venous connections to the cavernous sinus (via the superior/inferior ophthalmic veins, and indirectly via the facial vein/angular vein pathway), orbital cellulitis (often secondary to ethmoidal sinusitis spreading through the thin medial orbital wall) carries a risk of progressing to septic cavernous sinus thrombosis: presenting with the combined deficit pattern of CN III, IV, V1, V2, VI dysfunction, along with proptosis, chemosis, and systemic sepsis, a genuine ophthalmological and neurological emergency.

Central retinal artery occlusion anatomy:

• Given the central retinal artery's function as a true end-artery with essentially no collateral circulation, its acute occlusion (embolic, most commonly from carotid or cardiac sources) causes sudden, painless, profound monocular vision loss, a genuine ophthalmological emergency, since retinal tissue tolerates ischaemia extremely poorly (similar in principle to other end-artery-dependent tissues covered throughout this Atlas, such as the appendix or testis) and the window for any potentially vision-salvaging intervention is very narrow.