What are the seven bones that form the orbit?

The seven orbital bones are the frontal (roof), zygomatic (lateral wall and floor), maxillary (floor and medial wall), ethmoid (medial wall), sphenoid (lateral wall and apex), palatine (posterior floor), and lacrimal (medial wall). The medial wall, formed largely by the ethmoid bone, is extremely thin and is often called the lamina papyracea (paper layer). It is the most commonly fractured wall in orbital blowout fractures.

What is epiphora and what causes it?

Epiphora is excessive tearing or overflow of tears onto the cheek. It occurs when tear production exceeds the drainage capacity or when the drainage system is obstructed. Causes include punctal stenosis (narrowing of the tear duct opening), canalicular obstruction (often from infection or trauma), lacrimal sac obstruction, or nasolacrimal duct obstruction. In newborns, congenital nasolacrimal duct obstruction is the most common cause and usually resolves spontaneously or with massage by 12 months of age.

What cranial nerve palsy causes a drooping eyelid (ptosis)?

Ptosis (drooping of the upper eyelid) can result from CN III (oculomotor nerve) palsy, which paralyzes the levator palpebrae superioris muscle. This causes complete ptosis along with an eye that is down and out, a dilated pupil (due to loss of parasympathetic tone), and inability to move the eye in most directions. Mild ptosis can also result from Horner's syndrome (loss of sympathetic innervation to Muller's muscle), myasthenia gravis, or mechanical causes.

What is a Meibomian gland and what happens when it becomes blocked?

Meibomian glands are sebaceous glands embedded in the tarsal plates of both eyelids. There are approximately 25 to 40 in the upper lid and 20 to 30 in the lower lid. They secrete the oily (lipid) layer of the tear film, which reduces evaporation. When a Meibomian gland becomes blocked, it can form a chalazion, a chronic inflammatory granuloma that appears as a firm, painless lump in the lid. Meibomian gland dysfunction is a major cause of evaporative dry eye.

Why does blowing the nose help after crying?

Tears drain from the puncta through the canaliculi, lacrimal sac, and nasolacrimal duct into the inferior meatus of the nose. During excessive tearing (crying), more tears drain into the nose than usual, causing nasal congestion and a runny nose. Blowing the nose helps clear the nasal mucus and allows more continuous tear drainage. This anatomical connection is also why eye drops instilled topically drain into the nose and can cause systemic effects (e.g., beta-blocker eye drops for glaucoma can cause systemic bradycardia).

Orbit & Adnexa Anatomy for the CPO Exam

The orbit is the bony protective socket that houses the eyeball and its supporting structures. The adnexa, meaning the accessory structures around the eye, include the eyelids, lacrimal apparatus, conjunctiva, and the extraocular muscles. Understanding these structures is essential for the CPO exam and for recognizing conditions that present in the periorbital region.

The Bony Orbit

The orbit is a pyramidal cavity formed by seven bones of the skull. A useful mnemonic to remember them is "My Friend Eats Zucchini Pie, Sam Loves":

Maxillary bone (floor and medial wall)
Frontal bone (roof)
Ethmoid bone (medial wall)
Zygomatic bone (lateral wall and floor)
Palatine bone (small portion of floor)
Sphenoid bone (lateral wall and apex)
Lacrimal bone (medial wall)

The orbit is widest near its opening (orbital rim) and tapers to the optic canal at the apex, through which the optic nerve exits to reach the brain. The medial wall (primarily the ethmoid bone) is very thin ("paper-plate thin") and commonly fractured in blunt trauma, allowing orbital fat to herniate into the ethmoid sinus.

A "blowout fracture" of the orbital floor (maxillary bone) is a common injury from blunt trauma to the eye. The globe is pushed inward, increasing pressure in the orbit, which forces the thin orbital floor to fracture downward. Orbital fat and sometimes the inferior rectus muscle can become entrapped, causing diplopia on upward gaze.

The Eyelids

The eyelids (palpebrae) protect the eye, distribute tears across the cornea with each blink, and limit light entry during sleep. Each eyelid consists of multiple layers:

Skin: The thinnest skin in the body, which allows for easy swelling.
Orbicularis oculi muscle: The sphincter muscle of the eyelid that closes the lids, innervated by cranial nerve VII (facial nerve).
Tarsal plate: The rigid fibrous skeleton that gives each lid its shape. It contains Meibomian glands, which secrete the oily outer layer of the tear film.
Conjunctiva: The thin mucous membrane lining the inner eyelid surface (palpebral conjunctiva) and covering the anterior sclera (bulbar conjunctiva).

The levator palpebrae superioris muscle (innervated by CN III) elevates the upper eyelid. Weakness or paralysis of this muscle causes ptosis (drooping eyelid). Muller's muscle, innervated by the sympathetic nervous system, provides additional upper lid elevation; loss of sympathetic tone causes mild ptosis as seen in Horner's syndrome.

The Lacrimal Apparatus

The lacrimal system includes both the production and drainage of tears:

Tear Production

The main lacrimal gland is located in the superolateral orbit beneath the orbital rim. It produces the aqueous (watery) component of the tear film. Accessory lacrimal glands (Krause and Wolfring glands) are found in the conjunctival fornix and contribute to basal tear secretion. The Meibomian glands in the tarsal plates produce the lipid layer of tears, and the goblet cells of the conjunctiva produce the mucin layer.

Tear Drainage

Tears drain through the lacrimal drainage system:

Puncta: Two small openings (superior and inferior) at the medial corner of each eyelid margin.
Canaliculi: Small channels connecting the puncta to the lacrimal sac.
Lacrimal sac: Located in the medial canthal area; receives tears from the canaliculi.
Nasolacrimal duct: Carries tears from the lacrimal sac into the inferior meatus of the nasal cavity.

Obstruction anywhere along this pathway causes epiphora (overflow tearing). Congenital nasolacrimal duct obstruction is common in newborns and usually resolves with massage and maturation by 12 months.

Relevant Cranial Nerves

Several cranial nerves are critical for orbital and adnexal function:

Cranial Nerve	Name	Orbital Function
CN II	Optic nerve	Vision (exits via optic canal)
CN III	Oculomotor	Most extraocular muscles, upper lid elevation, pupil constriction
CN IV	Trochlear	Superior oblique muscle
CN V1/V2	Trigeminal (ophthalmic/maxillary branches)	Corneal sensation, eyelid and facial sensation
CN VI	Abducens	Lateral rectus muscle
CN VII	Facial	Orbicularis oculi (lid closure)

Key Takeaways

The orbit is formed by seven bones; the medial wall and floor are thin and prone to blowout fractures.
Eyelid closure is via the orbicularis oculi (CN VII); lid opening involves the levator palpebrae (CN III) and Muller's muscle (sympathetic).
The lacrimal system drains tears from puncta through canaliculi to lacrimal sac to nasolacrimal duct and into the nose.
Meibomian glands produce the oily tear film layer; goblet cells produce mucin; the lacrimal gland produces the aqueous layer.
CN III, IV, and VI control extraocular movements; CN VII controls lid closure; CN V provides corneal and facial sensation.