# Orbit Anatomy > Orbit anatomy for MBBS Anatomy: four walls, optic canal and orbital fissures contents, blow-out fracture patterns, lamina papyracea and applied clinical notes. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/orbit-anatomy - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Orbit Anatomy", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/orbit-anatomy ## Direct answer A four-sided pyramid bounded by seven bones opens onto the face as the orbital opening and narrows back to the optic canal, which transmits the optic nerve and ophthalmic artery. The superior orbital fissure between the greater and lesser wings of the sphenoid carries the oculomotor, trochlear, abducent and three branches of V1 with the superior ophthalmic vein; the inferior orbital fissure carries the maxillary nerve's infraorbital and zygomatic branches. The floor is the thinnest wall and the medial wall — the lamina papyracea of the ethmoid — the weakest, facts that dictate the patterns of blow-out fracture and the spread of ethmoid sinusitis into the orbit. ## What you must remember - Roof: orbital plate of the frontal bone and lesser wing of sphenoid; the lacrimal gland sits in its anterolateral fossa. - Floor: orbital plate of the maxilla, zygomatic bone and palatine orbital process; the infraorbital groove and canal run in it, carrying the infraorbital nerve. - Medial wall: frontal process of maxilla, lacrimal bone, orbital plate (lamina papyracea) of the ethmoid and body of sphenoid; the lacrimal fossa for the sac lies in front. - Lateral wall: zygomatic bone and greater wing of sphenoid — the strongest wall, farthest from the orbital apex. - Optic canal: optic nerve with its meningeal sheaths and the ophthalmic artery, plus sympathetic fibres. - Superior orbital fissure contents: oculomotor (superior and inferior divisions), trochlear, lacrimal, frontal and nasociliary nerves, abducent nerve, superior ophthalmic vein — the mnemonic "Lucy Thompson's Nice English Accent" region-wise. - The orbit communicates with the nose through the nasolacrimal canal, with the infratemporal fossa through the inferior orbital fissure, and with the cranial cavity through the optic canal and superior orbital fissure. ## Working through a blow-out fracture of the orbital floor A cricket ball strikes a young man's right eye; the globe, resistant to rupture, transmits the force to the thin orbital walls. Presenting features after the swelling settles are diplopia on looking up, anaesthesia of the cheek and upper lip, and enophthalmos. Map each sign to a wall. The floor cracks — the orbital plate of the maxilla is paper-thin behind its reinforced anterior margin — and the inferior rectus and orbital fat herniate into the maxillary sinus; the tethered muscle fails to elevate the globe, giving restricted upgaze and positive forced duction testing. The infraorbital nerve in its canal within the floor is contused, producing the cheek and lip numbness; late enophthalmos follows loss of orbital fat volume into the sinus. Work the imaging and plan. A coronal CT shows the hanging drop of soft tissue in the roof of the maxillary sinus and fractures of the medial wall too, because lamina papyracea involvement is frequent — orbital emphysema after nose blowing is its signature. Surgical repair per current ophthalmic practice is timed by muscle involvement: early repair for radiological muscle entrapment with diplopia, planned reconstruction with an implant for large floor defects threatening enophthalmos. In children, a trapdoor greenstick fracture may incarcerate the muscle with an oculocardiac reflex — bradycardia and vomiting on eye movement — demanding urgent release, a paediatric pattern worth remembering separately. ## Where students slip The fissure contents are jumbled in most first attempts. The reliable scheme is to divide the superior orbital fissure by the common tendinous ring of Zinn: within the ring run the oculomotor divisions, the nasociliary nerve and the abducent nerve (also the optic canal's contents just above); outside the ring, in the lateral part, run the lacrimal, frontal and trochlear nerves with the superior ophthalmic vein — which is why the "lacrimofrontotrochlear" trio escapes an annulus-level lesion. Second, students attribute blow-out fracture to "fracture of the strongest wall"; it is the floor and medial wall that give way, the lateral wall and roof being buttressed. Third, remember the orbit's neighbouring spaces downward (maxillary sinus), medial (ethmoids), above (frontal sinus and anterior cranial fossa) — one question on "where does orbital infection drain" answers itself from this neighbourhood map, and ethmoiditis in a child reaching the orbit through lamina papyracea is the textbook route of paediatric orbital cellulitis. ## Frequently asked questions ### Which is the thinnest wall of the orbit, and which the weakest? The floor (orbital plate of the maxilla) is thinnest posteriorly; the medial wall's lamina papyracea of the ethmoid is the weakest and most frequently fractured in blow-out injuries. ### Which structures pass through the optic canal? The optic nerve with its sheaths, the ophthalmic artery and sympathetic nerve fibres. ### What travels within the common tendinous ring in the superior orbital fissure? The superior and inferior divisions of the oculomotor nerve, the nasociliary nerve and the abducent nerve. ### Why does a blow-out fracture cause cheek numbness? The infraorbital nerve runs in the infraorbital canal within the orbital floor and is injured when the floor fractures. ### How does ethmoid sinusitis spread to the orbit? Directly through the paper-thin lamina papyracea forming the medial orbital wall — the commonest route of orbital cellulitis in children.