Which bones form the orbit?

Seven bones contribute to the orbital walls: frontal (roof), zygomatic (lateral wall), maxillary (floor), lacrimal (medial wall), ethmoid (medial wall, the thin lamina papyracea), sphenoid (apex and parts of lateral wall and roof), and palatine (a small contribution to the floor). The floor (maxillary) and medial wall (ethmoid) are the thinnest.

How do tears drain from the eye?

Tears drain through the puncta (small openings on the medial lid margins) into the canaliculi (short channels), then into the lacrimal sac (at the medial canthus), and finally through the nasolacrimal duct into the nose (inferior meatus). Blockage at any point causes epiphora (excessive tearing).

What happens in Bell's palsy affecting the eye?

Bell's palsy affects CN VII (facial nerve), which controls the orbicularis oculi muscle. The patient cannot fully close the affected eye (lagophthalmos). This leads to corneal exposure, drying, and potential ulceration. Treatment includes frequent artificial tears, lubricating ointment at night, and sometimes taping the lid closed during sleep to protect the cornea.

Orbit and Adnexa: Bony Orbit, Eyelids, Lacrimal System, and Cranial Nerves

The Bony Orbit

The orbit is a pyramidal bony socket that houses and protects the globe, extraocular muscles, nerves, blood vessels, and orbital fat. Seven bones contribute to its walls: frontal, zygomatic, maxillary, lacrimal, ethmoid, sphenoid, and palatine.

Key orbital landmarks:

Orbital rim: The sturdy bony edge that protects the globe from direct frontal trauma
Orbital floor: The thinnest wall, most susceptible to blowout fractures from blunt trauma. The infraorbital nerve and canal run through it.
Medial wall: Also thin (lamina papyracea of the ethmoid bone). Adjacent to the ethmoid sinuses.
Optic canal: Transmits the optic nerve and ophthalmic artery into the orbit from the cranial cavity.
Superior orbital fissure: Transmits CN III, IV, VI, and the ophthalmic division of CN V into the orbit.

The Eyelids

The eyelids protect the eye, distribute the tear film with each blink, and regulate light entry. Key structures include:

Orbicularis oculi: The circular muscle that closes the eyelids. Innervated by CN VII (facial nerve).
Levator palpebrae superioris: The primary muscle that opens (elevates) the upper lid. Innervated by CN III (oculomotor nerve).
Muller's muscle: A smooth muscle that provides additional upper lid elevation. Innervated by the sympathetic nervous system. Loss of sympathetic innervation (Horner's syndrome) causes mild ptosis.
Tarsal plates: Dense connective tissue that provides structural support to the lid. The meibomian glands are embedded within the tarsal plates and produce the lipid layer of the tear film.

The Lacrimal System

Tear Production

The lacrimal gland is located in the superotemporal orbit and produces the aqueous (watery) component of tears through reflex and basal secretion. Accessory lacrimal glands (glands of Krause and Wolfring) contribute to basal tear production.

Tear Drainage

Tears drain through a series of structures:

Puncta: Small openings on the medial aspect of each upper and lower lid margin
Canaliculi: Short channels connecting the puncta to the lacrimal sac
Lacrimal sac: Collects tears at the medial canthus
Nasolacrimal duct: Drains tears from the lacrimal sac into the inferior nasal meatus

Blockage at any point in this pathway causes excessive tearing (epiphora). In infants, a congenitally blocked nasolacrimal duct is the most common cause.

When a patient presents with chronic tearing, determine whether the cause is overproduction (reflex tearing from dry eye or irritation) or impaired drainage (nasolacrimal obstruction). The treatment approach differs significantly. Dye disappearance test and probing can help differentiate.

Cranial Nerves Relevant to the Orbit

Cranial Nerve	Name	Function
CN II	Optic	Vision (afferent pathway for light perception)
CN III	Oculomotor	Most EOMs, levator, pupil constriction, accommodation
CN IV	Trochlear	Superior oblique muscle
CN V	Trigeminal	Corneal and facial sensation; V1 branch serves the orbit
CN VI	Abducens	Lateral rectus muscle
CN VII	Facial	Orbicularis oculi (lid closure), lacrimal gland secretion

Confusing the nerve that opens the eye (CN III, levator) with the nerve that closes the eye (CN VII, orbicularis). A CN III palsy causes ptosis (lid droop) because the levator cannot lift the lid. A CN VII palsy causes lagophthalmos (inability to close the eye), leading to exposure keratopathy.

Key Takeaways

The orbit is formed by seven bones; the floor and medial wall are the thinnest and most fracture-prone
The levator (CN III) opens the upper lid; the orbicularis oculi (CN VII) closes the eyelids
The lacrimal gland produces tears that drain through the puncta, canaliculi, lacrimal sac, and nasolacrimal duct
Six cranial nerves serve the orbit: CN II (vision), III (motor), IV (superior oblique), V (sensation), VI (lateral rectus), VII (lid closure and tears)
Orbital floor fractures and nasolacrimal duct obstruction are common clinical conditions related to orbital anatomy