eyelid

The eyelids are two movable folds that protect the surface of the eye and help regulate tear distribution. Each lid comprises skin, subcutaneous tissue, the orbicularis oculi muscle, a dense tarsal (tarsal) plate, and a conjugate lining on the inner surface. The lids meet at the medial and lateral canthi, forming the palpebral fissure when the eye is open. The upper lid is positioned by the levator palpebrae superioris, with the tarsal border maintained by Mueller’s muscle, while the lower lid relies mainly on the inferior retractors and surrounding tissues.

Functions include protection from mechanical injury, redistribution of the tear film across the ocular surface during blinking, and facilitation of tear drainage. Blink movements, typically about 15 to 20 times per minute, spread tears and remove debris, while the lid margins and associated glands contribute to tear film stability and lubrication.

The margin features eyelashes and a complex set of glands. Meibomian glands lie within the tarsal plate and secrete lipid to reduce tear evaporation; sebaceous Zeis glands accompany the eyelashes; Moll glands provide apocrine secretions. The inner surface is lined by conjunctiva, which contains goblet cells that produce mucous to supplement the tear film.

Innervation and blood supply support lid function. Motor control of the orbicularis oculi is primarily by the facial nerve (cn VII), while eyelid elevation is driven by the oculomotor nerve (cn III) via the levator, with sympathetic input to Müller's muscle. Blood supply comes from branches of the facial and ophthalmic arteries, and lymphatics drain to preauricular and other nearby nodes.

Common conditions include ptosis (drooping eyelid), blepharitis (inflammation of the lid margins), styes or chalazia (infections or granulomas of glands), and eyelid malpositions such as entropion, ectropion, or lagophthalmos.