Eyelid

Recommended video: Eyelids and tunica conjunctiva [21:20]

Sagittal section of the orbital cavity and eyeball.

The eyelids are thin soft tissue structures that cover and protect the anterior surface of the eyeball. Their anatomy is rather complex, comprising of skin, muscles, nerves and blood vessels.

The eyelids are divided into superior and inferior portions, converging at the medial and lateral canthi (corners) of the eye. 

The eye has two palpebral fissures, horizontal and vertical, which stand for the normal exposed area between the superior and inferior eyelids. The elliptical opening between them is known as the palpebral aperture. 

The eyelids are divided into three anatomical lamellae:

• The anterior lamella is vascularized and consists of skin and the orbicularis oculi muscle. The eyelashes are attached to this superficial layer of skin and subcutaneous tissue along with the ciliary glands of Moll (sweat glands) and the sebaceous glands (glands of Zeis).
• The middle lamella provides structural support and consists of the orbital septum, which arises from the orbital periosteum, and orbital and septal fat.
• The posterior lamella provides mucosal lining to the eyelids and consists of the tarsal plates (superior and inferior), the conjunctiva and the superior eyelid muscle retractors (levator palpebrae superioris and superior tarsal muscle). The tarsal plates are the main scaffolding of the eyelids and are made of dense connective tissue. They also contain the eyelash follicles and the tarsal glands (otherwise known as Meibomian glands) secreting an oily substance slowing the evaporation of the eye’s tear film. The conjunctiva is a thin mucous membrane reflecting onto the sclera of the eyeball. It is the deepest layer of the eyelid.

The eyelids receive arterial supply from the ophthalmic artery, the facial artery and the superficial temporal artery. Venous drainage is achieved via the medial and lateral palpebral veins.

Sensory innervation is provided by branches of the trigeminal nerve (CN V), specifically the ophthalmic nerve (CN V1) and the maxillary nerve (CN V2).

Functionally, eyelids assist in regulating the amount of light reaching the eye, maintaining and distributing the eye's tear film during blinking and maintaining tear flow, by acting on the conjunctival and lacrimal sacs.