21st February 2024

The Eyelid

The eyelids are thin and movable layers that envelop the front part of the eyeball. They serve as protection against excessive light and injury while ensuring adequate lubrication by spreading tears across the eye’s surface.

The eyelids are divided into upper and lower sections, which come together at the medial canthus and lateral canthus of the eye. The palpebral aperture refers to the opening between these upper and lower sections. The important structures of the eyelid are as follows.

Skin and Subcutaneous Tissue

The most superficial layer of the eyelid consists of the skin and subcutaneous tissue. The skin in this region is one of the thinnest on the human body. Within the subcutaneous layer, there is loose connective tissue but no subcutaneous fat. Consequently, the eyelids can easily swell due to oedema or blood accumulation. The eyelashes, along with the modified sweat glands (ciliary glands of Moll) and sebaceous glands (glands of Zeiss), are found here. 

Orbicularis Oculi

The Orbicularis Oculi muscle encircles the eye and has 3 parts: orbital, which forcefully closes the eyelid; palpebral, which gently closes the eyelids; and lacrimal, which drains tears. It is responsible for blinking and maintaining closure to protect the eye. It originates from the medial orbital margin and inserts into the skin around the orbit, as well as the superior and inferior tarsal plates. It is innervated by the temporal and zygomatic branches of the facial nerve. 

Tarsal Plates

The tarsal plate is a dense strip of connective tissue within each eyelid. It provides structural support, ensuring they maintain their shape and position over the eye. The superior tarsus (upper eyelid) and inferior tarsus (lower eyelid) are located deep to the palpebral region of the Orbicularis Oculi. The Meibomian glands found within the tarsal plate secrete oil to the eye’s surface to slow the evaporation of the tear film and prevent the eyelids from sticking together when shut. These glands can become blocked, leading to blepharitis and stye and chalazion formation.

Levator Palpebrae Superioris (LPS)

The LPS, supplied by the superior branch of the oculomotor nerve, works to open the eyelid. It originates from the lesser wing of the sphenoid and inserts into the upper eyelid and superior tarsal plate. Muller’s muscle assists the LPS in opening the eyelid and is supplied by the sympathetic fibres from the superior cervical ganglion.

Innervation and Vasculature of the Eyelid

The eyelid is supplied by the branches of the trigeminal nerve, with V1 supplying the upper eyelid and V2 supplying the lower eyelid. The ophthalmic, facial and superficial temporal artery supply the eyelid, and it is drained by a rich network of veins split medially and laterally. Medially, it drains via the medial palpebral vein into the angular and ophthalmic veins. Laterally, blood drains from the lateral palpebral vein to the superficial temporal vein.