C

— Medial canthus

Pupil

Lower eyelid

Iris

Limbus

The conjunctiva is a clear mucous membrane with two easily visible components. The bulbar conjunctiva covers most of the anterior eyeball, adhering loosely to the underlying tissue. It meets the cornea at the limbus. The palpebral conjunctiva lines the eyelids. The two parts of the conjunctiva merge in a folded recess that permits the eyeball to move.

Within the eyelids lie firm strips of connective tissue called tarsal plates. Each plate contains a parallel row of meibomian glands, which open on the lid margin. The levator palpebrae muscle, which raises the upper eyelid, is innervated by the oculomotor nerve (Cranial Nerve III). Smooth muscle, innervated by the sympathetic nervous system, contributes to raising this lid.

A film of tear fluid protects the conjunctiva and cornea from drying, inhibits microbial growth, and gives a smooth optical surface to the cornea. This fluid comes from three sources: meibomian glands, conjunctival glands, and the lacrimal gland. The lacrimal gland lies mostly within the bony orbit, above and lateral to the eyeball. The tear fluid spreads across the eye and drains medially through two tiny holes called lacrimal puncta. The tears then pass into the lacrimal sac and on into the nose through the nasolacrimal duct. (You can easily find a punctum atop the small elevation of the lower lid medially. You cannot detect the lacrimal sac, which rests in a small depression inside the bony orbit.)

The eyeball is a spherical structure that focuses light on the neurosensory elements within the retina. The muscles of the iris control pupillary size. Muscles of the ciliary body control the thickness of the lens, allowing the eye to focus on near or distant objects.

Levator palpebrae

Levator palpebrae

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