Eyelids

The eyelids are mobile mucocutaneous folds designed to protect the eye from external aggression. They can temporarily cover the eye more or less completely. We generally distinguish between an upper eyelid, an inferior eyelid and the third eyelid (or semi-lunar fold).

When the eyelids are open, an elliptical space, the palpebral fissure, is left between their edges, whose angles correspond to the junctions of the upper and lower eyelids and are called palpebral commissures. The third eyelid is located in the medial angle of the eye, behind the other 2 eyelids.

The upper and lower eyelids close to cover the free surface of the eyeball. They play a role in protecting the eye from mechanical aggression and light overexposure, and their blinking prevents the eye from drying out. The structure of the upper and lower eyelids is similar, while that of the third eyelid is different. Their structure is divided into 3 layers:

• The anterior surface is a skin covering.

• The middle layer is fibro-muscular, including the orbital septum, tarsus, palpebral ligaments, tarsal muscles, orbicularis oculi muscle fibers and the muscle levator palpebrae superioris.

• The posterior surface is covered by the palpebral conjunctiva.

The third eyelid, on the other hand, is entirely covered by conjunctiva, contains no muscle fibers, and its structure is maintained by a cartilage to which an accessory lacrimal gland is attached.

The eyelids contain a large number of structures related to the lacrimal apparatus, such as tarsal glands, lacrimal punctum and accessory lacrimal glands, and are therefore involved in the production and flow of tears.