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 |
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
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
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