eyelids

The eyelids are paired movable folds of skin that protect and moisten the eyes. Each lid consists of skin, subcutaneous tissue, the orbicularis oculi muscle, a dense fibrous tarsal plate that provides shape, and a palpebral conjunctiva lining the inner surface. The upper eyelid is raised by the levator palpebrae superioris muscle, with assistance from the smooth muscle known as Müller's muscle, while the lower lid is moved mainly by the orbicularis oculi and supporting tissues. The eyelid margin also hosts eyelashes and numerous glands.

Functionally, eyelids protect the ocular surface from debris, light, and wind. They blink reflexively and voluntarily to spread a thin tear film across the cornea, maintaining lubrication, removing particles, and aiding tear drainage. Blinking also helps seal the eye during sleep and reduces evaporative loss of tears.

Glands and border anatomy contribute to tear film health. Meibomian glands within the tarsal plates secrete lipids that stabilize the tear film’s outer layer. Glands of Zeis and Moll surround the eyelashes and contribute sebum and sweat secretions. The inner surface of the lids is lined by palpebral conjunctiva, which continues with the bulbar conjunctiva over the eyeball. The tear film is supplied by the lacrimal glands and drained away through puncta and canaliculi into the nasolacrimal duct.

Nerve supply and control involve the facial nerve, which powers the orbicularis oculi, and the oculomotor nerve, which elevates the upper lid via the levator palpebrae superioris (with sympathetic input to Müller's muscle). Common eyelid conditions include blepharitis (inflammation), ptosis (drooping), entropion (inward turning), ectropion (outward turning), and Meibomian gland dysfunction.