Eyelid Structure: Anterior and Posterior Surfaces
Histologically, the eyelid is a layered structure clearly demarcated into an anterior cutaneous part, a posterior conjunctival part, and a supporting musculoglandular framework.
The anterior surface (cutaneous part) is formed by thin skin consisting of standard layers: epidermis and dermis. Numerous cutaneous appendages are located here:
- Fine vellus hairs grow across the general surface, while larger hairs—eyelashes—are located along the free margin of the eyelid.
- Sebaceous glands, characterized by holocrine secretion, are associated with the roots of the eyelashes.
- Sweat glands are represented by two types: apocrine glands located immediately adjacent to the eyelashes, and merocrine glands distributed evenly across the rest of the eyelid skin.
The posterior surface (conjunctival part) is closely lined by the mucous membrane. It covers the inner aspect of the eyelid, smoothly transitions onto the eyeball at the fornix, and this epithelial layer then continues directly into the corneal epithelium. Histologically, the posterior surface consists of two components:
- Epithelium: Non-keratinized stratified squamous.
- Lamina propria: Formed by loose connective tissue. Its most important distinguishing feature is extremely rich vascularization with a dense capillary network.
Internal Eyelid Framework and Muscular Apparatus
The inner part of the eyelid performs supporting and motor functions, and also houses specific glandular structures.
The tarsal plate (so-called eyelid cartilage) serves as the structural foundation. Despite its traditional name, it is composed of dense connective tissue and is merely a histological vestige of cartilage. Within this plate, oriented perpendicularly to the eyelid margin, lie the Meibomian glands (averaging about 30 in number).
Morphologically, the Meibomian glands are modified sebaceous glands. They are simple branched tubuloalveolar glands with a holocrine secretion type. The excretory ducts of these glands open onto the eyelid margin posterior to the lash line.
Between the skin and the tarsal plate lies the muscular apparatus, formed by bundles of skeletal muscle tissue:
- Levator palpebrae superioris: Its bundles are oriented vertically (present in the upper eyelid).
- Orbicularis oculi: Represented by circular muscle bundles running near the free margin of both eyelids.
Additionally, small accessory lacrimal glands are hidden beneath the conjunctiva in the eyelid fornix, contributing to constant eye lubrication.
Lacrimal Apparatus (*Apparatus lacrimalis*)
The lacrimal apparatus includes structures responsible for extensive tear secretion and its continuous drainage pathways.
As the main lacrimal gland (Glandula lacrimalis) is localized in the superolateral angle of the orbit. By morphological classification, it is a compound tubuloalveolar gland. It actively produces tear fluid, whose chemical composition includes 1.5% sodium chloride, 0.5% albumin, and a minor amount of mucus.
Lacrimal drainage pathways ensure the removal of the secretion. The drainage system originates in the medial angle of the orbit and includes a strictly defined sequence of anatomical structures:
- Lacrimal canaliculi (superior and inferior).
- Lacrimal sac.
- Nasolacrimal duct, which opens into the inferior nasal meatus, discharging fluid into the nasal cavity.
Histologically, the walls of the lacrimal sac and nasolacrimal duct are lined by stratified columnar and pseudostratified epithelium, reflecting their fluid-conveying function.