Structure and Function of the External Eye and Cornea

A set of vocabulary flashcards covering the anatomy and physiology of the eyelids, conjunctiva, sclera, limbus, and various layers of the cornea based on lecture materials.

Cilia

The eyelashes of the eyelid that catch small particles and act as sensors to stimulate reflex eyelid closure.

Orbicularis oculi

The muscle innervated by $CN\,VII$ that is responsible for closing the upper and lower eyelids.

Levator palpebrae superioris

The muscle innervated by $CN\,III$ that inserts into the tarsal plate and skin to elevate the upper eyelid.

Müller muscle

A muscle innervated by sympathetic nerves that contributes to the elevation of the upper eyelid.

Glands of Zeis

Numerous modified sebaceous glands located near the eyelid margin.

Glands of Moll

Apocrine sweat glands located near the eyelid margin.

Meibomian glands

Holocrine sebaceous glands that secrete meibum to prevent tear evaporation and maintain the lipid layer of the tear film.

Lacrimal Functional Unit (LFU)

A system comprising the lacrimal glands, ocular surface (conjunctiva and cornea), eyelids, and the sensory and motor nerves connecting them.

Tear Film

A liquid layer consisting of soluble mucus, aqueous, and liquid lipids/proteins that provides lubrication, refractive power, and antimicrobial defense.

Glycocalyx

The outermost surface of the superficial corneal epithelial cells characterized by microvilli and membrane-associated mucins ($MUC\,1, 4, \text{ and } 16$).

Palpebral conjunctiva

The geographic zone of the conjunctiva that lines the inner eyelid and is tightly adherent to the underlying tarsus.

Bulbar conjunctiva

The geographic zone of the conjunctiva that covers the ocular surface, is loosely attached to the Tenon capsule, and inserts into the limbus.

Forniceal conjunctiva

The geographic zone of the conjunctiva that covers the superior and inferior fornices.

Glands of Krause

Accessory lacrimal glands located in the fornix of the palpebral conjunctiva.

Glands of Wolfring

Accessory lacrimal glands located in the subconjunctival tissue near the upper border of the superior tarsal plate.

Plica semilunaris

A crescent-shaped vertical fold of conjunctiva located at the medial angle of the eye.

Caruncle

A fleshy, ovoid mass ($5\,mm$ high by $3\,mm$ wide) at the medial angle containing goblet cells, lacrimal tissue, hairs, and glands.

Substantia propria

The layer of the conjunctiva consisting of loose connective tissue and conjunctiva-associated lymphoid tissue ($CALT$).

Sclera

The opaque, white, fibrous tissue extending from the limbus to the optic nerve, divided into the episclera, stroma, and lamina fusca.

Limbus

The transition zone between the transparent cornea and opaque sclera that harbors corneal epithelial stem cells.

Palisades of Vogt

Radially oriented fibrovascular ridges along the limbus thought to be the niche for limbal stem cells.

XYZ hypothesis

A theory of corneal renewal where $X$ is proliferation, $Y$ is centripetal migration, and $Z$ is desquamation of superficial cells.

Bowman layer

An acellular condensate of irregularly arranged collagen fibrils, $8-15\,\mu m$ thick, situated beneath the corneal epithelium.

Keratocytes

Flattened fibroblasts located between stromal collagen lamellae that continually digest and manufacture stromal molecules.

Dua layer (Pre-Descemet layer)

A strong, acellular layer approximately $15\,\mu m$ thick located between the stroma and the Descemet membrane.

Descemet membrane

The basement membrane of the corneal endothelium, consisting of an anterior banded zone and a posterior nonbanded zone.

Schwalbe line

A gonioscopic landmark defining the termination of the Descemet membrane and the beginning of the trabecular meshwork.

Corneal endothelium

A single layer of mostly hexagonal cells ($4-6\,\mu m$ thick) that maintains corneal transparency via a metabolic pump and barrier function.

Polymegethism

Clinical variability in the size of corneal endothelial cells, often observed via specular microscopy.

Polymorphism

Clinical variability in the shape of corneal endothelial cells.

Langerhans cells

A subclass of dendritic antigen-presenting cells ($APCs$) found in the corneal and conjunctival epithelium.

Corneal vertex

The point at the intersection of the line of sight and the corneal surface, often represented by the corneal light reflex.

Corneal apex

The point of maximum curvature on the cornea, typically located temporal to the center of the pupil.