What muscles open and close the eyelids?

The orbicularis oculi muscle (innervated by CN VII, facial nerve) closes the eyelids. The levator palpebrae superioris (innervated by CN III, oculomotor nerve) opens the upper eyelid. Muller's muscle (sympathetic innervation) assists the levator in maintaining upper lid tone.

What are the meibomian glands?

Meibomian glands (also called tarsal glands) are sebaceous glands embedded in the tarsal plates of the eyelids. There are about 25-30 in the upper lid and 20-25 in the lower lid. They secrete the lipid (oil) layer of the tear film, which prevents tear evaporation.

Eyelid Anatomy & Adnexa for ABO

Q: What is the difference between a hordeolum and a chalazion?

A hordeolum (stye) is an acute bacterial infection of a lid gland, presenting as a painful, red, swollen bump. A chalazion is a chronic granulomatous inflammation of a blocked meibomian gland, presenting as a painless, firm nodule. A hordeolum may resolve with warm compresses and antibiotics, while a chalazion may require incision and drainage.

Eyelid Structure

The eyelids (palpebrae) are mobile folds of tissue that protect the anterior surface of the eye. They spread the tear film, block foreign objects, and shield the eye from excessive light. Each eyelid has a layered structure from outside to inside:

Skin: The thinnest skin in the body, with loose subcutaneous tissue
Orbicularis oculi muscle: The circular muscle responsible for eyelid closure (innervated by CN VII, the facial nerve)
Orbital septum: A fibrous sheet separating superficial from deeper structures
Tarsal plate: A dense connective tissue "skeleton" providing structural rigidity (about 25 mm wide, 10 mm tall in the upper lid)
Palpebral conjunctiva: The mucous membrane lining the inner eyelid surface

Key Eyelid Muscles

Muscle	Action	Innervation
Orbicularis oculi	Closes the eyelids (voluntary and reflex blink)	CN VII (facial nerve)
Levator palpebrae superioris	Opens the upper eyelid (voluntary elevation)	CN III (oculomotor)
Muller's muscle (tarsal muscle)	Assists upper lid elevation (maintains tone)	Sympathetic nervous system

The orbicularis oculi (lid closure) is innervated by CN VII. The levator (lid opening) is innervated by CN III. A CN VII palsy causes inability to close the eye (lagophthalmos). A CN III palsy causes ptosis (drooping upper lid). These are different nerves with opposite effects.

Eyelid Glands

Meibomian Glands (Tarsal Glands)

About 25-30 in the upper lid and 20-25 in the lower lid, embedded within the tarsal plates. They produce the lipid layer of the tear film. Their openings line the posterior lid margin just in front of the mucocutaneous junction (the gray line).

Glands of Zeis

Modified sebaceous glands associated with eyelash follicles. An infection of a Zeis gland produces an external hordeolum (stye).

Glands of Moll

Modified sweat glands along the lid margin, also associated with eyelash follicles.

Eyelid Margins and Landmarks

Anterior lamella: Skin and orbicularis muscle
Posterior lamella: Tarsal plate and conjunctiva
Gray line: The mucocutaneous junction at the lid margin, marking the boundary between anterior and posterior lamellae. Surgically important as a landmark.
Lid margin: Contains eyelash follicles (2-3 rows in the upper lid, 1-2 rows in the lower) and the meibomian gland orifices
Punctum: The small opening for tear drainage, located on the medial aspect of each lid margin

The Blink Mechanism

Normal blink rate is approximately 15-20 blinks per minute. During a blink, the upper lid sweeps downward and slightly nasally, spreading the tear film and pushing tears toward the puncta for drainage. The blink is important for:

Redistributing the tear film
Expressing meibomian gland secretions
Removing debris from the ocular surface
Stimulating reflex tear production

Patients who stare at screens frequently (digital device users) have a significantly reduced blink rate (sometimes as low as 3-4 blinks per minute). This incomplete blinking contributes to digital eye strain and evaporative dry eye. Recommending the 20-20-20 rule (every 20 minutes, look at something 20 feet away for 20 seconds) helps encourage more regular blinking.

Ocular Adnexa

The adnexa are the accessory structures surrounding the eye:

Orbit: The bony socket formed by seven skull bones that houses and protects the eye
Lacrimal apparatus: The lacrimal gland (produces tears) and drainage system (puncta, canaliculi, lacrimal sac, nasolacrimal duct)
Eyebrows: Deflect sweat and rain from the eyes
Eyelashes: Sensory hairs that trigger protective blink reflex when touched

Confusing a hordeolum (stye) with a chalazion. A hordeolum is an acute infection of a lid gland (Zeis or meibomian), presenting as a tender, red bump. A chalazion is a chronic, granulomatous inflammation of a blocked meibomian gland, presenting as a painless, firm nodule. Treatment approaches differ significantly.

Key Takeaways

Eyelids have layered structure: skin, orbicularis oculi, septum, tarsal plate, conjunctiva
Orbicularis (CN VII) closes the eye; levator (CN III) opens it
Meibomian glands in the tarsal plates produce the lipid layer of tears
Normal blink rate is 15-20 per minute; reduced during screen use
The gray line marks the mucocutaneous junction on the lid margin