Corpus: Eyelid

The eyelid is a tissue structure composed of muscles, connective tissue, skin, mucous membrane and glands, which serves to protect the eye by closing it.

2.1. Overview

Humans have two eyelids:

• The upper eyelid (palpebra superior)
• The lower eyelid (palpebra inferior)

The two eyelids meet medially and laterally, forming the boundaries of the medial and lateral corners of the eye, known as the internal and external canthus. The space between the eyelids is referred to as the palpebral fissure.

The palpebral fissure is bordered above and below by the palpebral edges (margines palpebrae). The anterior palpebral margin (limbus anterior palpebrae) transitions to the front of the eyelid, while the posterior margin (limbus posterior palpebrae) merges with the back of the eyelid, which is lined with conjunctiva.

The upper eyelid folds over at the conjunctival fornix, forming the upper eyelid skin crease, which runs parallel to the eyelid margin when the eye is open. The lower eyelid skin crease is usually only noticeable when looking downward.

2.2. Structure

The eyelid consists of an outer and an inner leaf.

• The outer leaf consists of eyelid skin the underlying facial muscles (mimic muscles) located in the subcutaneous tissue. These include the orbicularis oculi muscle and the superior levator palpebrae muscle, which together form the voluntary muscles of the eyelid.
• The inner leaf includes the orbital septum and the tarsal plate ("eyelid cartilage"), a dense connective tissue plate made of collagen and elastic fibers. The tarsus provides structural support to the eyelid and measures about 1 mm in thickness and 25 mm in width. Its height is approximately 10 mm in the upper eyelid and 5 mm in the lower eyelid. The superior and inferior tarsal muscles, composed of smooth muscle and innervated by the sympathetic nervous system, are embedded in the tarsus. The inner surface of the eyelid, which faces the eyeball, is lined with conjunctiva.

2.3. Glands

The eyelids contain several types of glands:

2.3.1. Zeis glands

The Zeis glands are located at the hair follicles of the eyelashes. They are sebaceous glands.

2.3.2. Krause glands

The Krause glands are accessory lacrimal glands that contribute to tear production.

2.3.3. Moll's glands

The minor glands are apocrine glands similar to sweat glands, with excretory ducts opening at the eyelid margin or the hair follicles of the eyelashes.

2.3.4. Meibomian glands

The Meibomian glands are found within the tarsus. Their ducts open near the posterior edge of the eyelid. These glands produce a lipid-rich secretion that forms the oily layer of the tear film, preventing the eye from drying out.

2.3.5. Wolfring glands

The Wolfring glands are small accessory tubuloalveolar lacrimal glands that are scattered along the upper edge of the eyelid cartilage and. Together with the Krause glands, they produce about 5 % of the tear film's basal secretion.

2.4. Blood supply

The eyelids are primarily supplied by the medial and lateral palpebral arteries, which anastomose to form the superior palpebral arch (upper eyelid) and the inferior palpebral arch (lower eyelid). These vessels originate as branches of the supraorbital artery and the lacrimal artery from the ophthalmic artery.

Additional contributions to the blood supply come from the superficial temporal artery, the angular artery, and the dorsal nasal artery.

2.5. Innervation

The eyelids receive sensory innervation via branches of the trigeminal nerve (cranial nerve V). The upper eyelid is innervated by the supratrochlear nerve, infratrochlear nerve, and lacrimal nerve, all branches of the ophthalmic nerve. The lower eyelid is innervated by the infraorbital nerve, a terminal branch of the maxillary nerve.

The external skin of the eyelids consists of multi-layered keratinized squamous epithelium with an underlying fat-free lamina propria. Eyelashes emerge from the eyelid margin. On the inner side, the eyelid transitions to a multi-layered columnar epithelium with goblet cells, which is part of the conjunctiva. The transition between the external skin and conjunctiva is made of parakeratinized squamous epithelium.

Eyelid closure can occur both voluntarily and reflexively. Closing the eyelids takes approximately 300 milliseconds, and the upper and lower eyelids move synchronously. The eyelids open more slowly than they close.

• The eyelids protect the eye from bright, dazzling light by tightly shutting, functioning as a glare shield. For sleeping, the eye can be closed light-tight by the eyelids.
• The eyelids close reflexively when an object approaches the eye, protecting the cornea from damage.
• Every blink spreads tear fluid across the cornea, preventing the eye from drying out.

Inflammation of the eyelids (blepharitis) is characterised by symptoms such as itching, burning, redness, and swelling of the eyelid margins (edema). However, eyelid edema may also occur in other conditions due to the delicate tissue’s tendency to swell.

Inflammation of the minor or Zeis glands is called a stye (hordeolum), while inflammation of the Meibomian glands is referred to as a chalazion.

Paralysis of the eyelid muscles can occur due to facial nerve or oculomotor nerve palsy, causing drooping of the eyelid (ptosis or lagophthalmos). In cases of peripheral facial nerve paralysis, the inability to close the eye can lead to corneal dryness.

Partial or complete absence of an eyelid due to trauma or tumors is called ablepharia.