Orbit

Orbita

Also known as: orbital cavity, eye socket

The pyramidal bony cavity (≈30 mL) formed by seven bones that houses and protects the eyeball, extraocular muscles, nerves, vessels, lacrimal gland and orbital fat.

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Overview

Volume≈30 mL (globe ≈7 mL)

Orbital opening≈35 mm high × 40 mm wide

Depth (rim to apex)≈40–45 mm

Orbital axis≈23° from the median plane

Each orbit is a four-sided pyramid with its base (orbital opening, ≈35 mm high × 40 mm wide) facing anterolaterally and its apex at the optic canal. The medial walls of the two orbits are nearly parallel, whereas the lateral walls are at about 90° to each other, so the axis of each orbit diverges ≈23° from the median plane — which is why the superior and inferior recti do not pull straight back along the visual axis.

The walls: roof — orbital plate of the frontal bone and lesser wing of the sphenoid (separates the orbit from the anterior cranial fossa); lateral wall — zygomatic bone and greater wing of the sphenoid (thickest and strongest); floor — maxilla, with zygomatic and palatine contributions (thin; traversed by the infraorbital groove and canal); medial wall — ethmoid (paper-thin lamina papyracea), lacrimal bone, frontal process of the maxilla and body of the sphenoid (thinnest wall).

The orbit communicates with the middle cranial fossa through the optic canal and superior orbital fissure, with the pterygopalatine and infratemporal fossae through the inferior orbital fissure, with the nasal cavity via the nasolacrimal canal, and with the face via the supraorbital and infraorbital foramina.

Location: Between the anterior cranial fossa above, the maxillary sinus below, the ethmoidal air cells and nasal cavity medially, and the temporal fossa laterally.

Parts & subdivisions

Optic canal
In the lesser wing of the sphenoid at the apex: transmits the optic nerve (with its meningeal sheaths) and the ophthalmic artery.
Superior orbital fissure
Between greater and lesser wings of the sphenoid: transmits oculomotor (III), trochlear (IV) and abducens (VI) nerves, the lacrimal, frontal and nasociliary branches of V1, the superior ophthalmic vein and sympathetic fibres. The common tendinous ring (annulus of Zinn) encircles its central part (III, nasociliary, VI pass within the ring; IV, frontal and lacrimal pass above it).
Inferior orbital fissure
Between greater wing of sphenoid and maxilla: transmits the infraorbital and zygomatic nerves (V2), infraorbital vessels and a communication of the inferior ophthalmic vein with the pterygoid plexus.
Lacrimal fossa and nasolacrimal canal
Fossa for the lacrimal gland in the superolateral roof (frontal bone); fossa for the lacrimal sac medially (lacrimal bone and maxilla) leading to the nasolacrimal canal.
Contents
Eyeball, optic nerve, six extraocular muscles and levator palpebrae superioris, ophthalmic artery and veins, branches of III, IV, V1, V2 and VI, ciliary ganglion, lacrimal gland, orbital fat and fascia (Tenon capsule, check ligaments, suspensory ligament).

Anatomy

Arterial supply: Ophthalmic artery (from the internal carotid) and the infraorbital artery (from the maxillary artery).
Venous drainage: Superior and inferior ophthalmic veins → cavernous sinus and pterygoid venous plexus; they are valveless, allowing spread of infection from the face.
Relations: Roof — anterior cranial fossa and frontal sinus; floor — maxillary sinus; medial wall — ethmoidal cells, sphenoid sinus, nasal cavity; lateral wall — temporal fossa and middle cranial fossa.

Function

Protect the eyeball and its neurovascular supply
Provide attachments for extraocular muscles and fascial suspension of the globe
Transmit nerves and vessels between the cranial cavity, face and pterygopalatine fossa

Clinical significance

Orbital blow-out fracture

A blow to the eye (e.g. a fist or ball) raises intraorbital pressure and fractures the thin floor (or medial wall). Orbital contents herniate into the maxillary sinus: inferior rectus entrapment (restricted upward gaze, vertical diplopia), enophthalmos, infraorbital nerve anaesthesia (cheek, upper lip) and the "teardrop sign" on CT. Medial wall fracture can cause orbital emphysema on nose-blowing.

Orbital cellulitis

Usually spreads from ethmoid sinusitis through the lamina papyracea. Unlike pre-septal cellulitis it causes proptosis, painful/restricted eye movements and reduced vision — an emergency; it can spread via valveless ophthalmic veins to cause cavernous sinus thrombosis.

Mnemonics

Bones of the orbit

"My Zany Sister Finds Excellent Lemon Pies" — Maxilla, Zygomatic, Sphenoid, Frontal, Ethmoid, Lacrimal, Palatine.

Superior orbital fissure contents

"Lazy French Tarts Sit Naked In Anticipation" — Lacrimal, Frontal, Trochlear (above the ring); Superior division of III, Nasociliary, Inferior division of III, Abducens (through the ring).

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.