The Cornea
The cornea is the transparent, avascular dome at the front of the eye. It provides approximately two-thirds of the eye's refractive power and serves as a protective barrier against infection and trauma. Its transparency depends on its precise structural organization, absence of blood vessels, and a functional endothelial pump.
The Five Corneal Layers
- Epithelium: The outermost layer, 5-6 cell layers thick. It is the cornea's primary barrier against infection. It regenerates rapidly (heals within 24-72 hours). Abrasions and ulcers involve damage to this layer.
- Bowman's layer: A thin, acellular layer of collagen. Once damaged, it does not regenerate and may form a scar.
- Stroma: The thickest layer, comprising about 90% of corneal thickness. Made of precisely arranged collagen fibrils whose regular spacing is essential for transparency. Stromal edema disrupts this arrangement and causes cloudiness.
- Descemet's membrane: A basement membrane produced by the endothelium. It thickens with age and can regenerate after injury.
- Endothelium: A single layer of cells on the innermost corneal surface. These cells actively pump fluid out of the stroma (the "endothelial pump"), maintaining corneal clarity. Endothelial cells do not regenerate in humans; when they are lost, the remaining cells spread to cover the gap.
Aqueous Humor Dynamics
Aqueous humor is produced by the ciliary body through active secretion and ultrafiltration. It serves multiple functions: nourishing the lens and posterior cornea (which lack blood vessels), maintaining intraocular pressure, and providing an optically clear medium.
The Flow Pathway
- Produced by the ciliary processes in the posterior chamber
- Flows through the pupil into the anterior chamber
- Drains through the trabecular meshwork at the anterior chamber angle
- Enters Schlemm's canal (a circumferential collector channel)
- Exits through collector channels into the episcleral veins
This conventional (trabecular) pathway accounts for approximately 80-90% of aqueous outflow. The remaining 10-20% drains through the uveoscleral pathway, passing through the ciliary body and sclera.
The Anterior Chamber Angle
The anterior chamber angle is formed where the peripheral iris meets the inner cornea. The key structures visible on gonioscopy, from posterior to anterior, are:
- Ciliary body band
- Scleral spur
- Trabecular meshwork (the primary drainage structure; posterior pigmented and anterior non-pigmented portions)
- Schwalbe's line (the termination of Descemet's membrane, marking the anterior boundary of the angle)
Key Takeaways
- The cornea has five layers; the endothelium is critical for transparency and does not regenerate
- The stroma accounts for 90% of corneal thickness; disrupted collagen arrangement causes cloudiness
- Aqueous humor is produced by the ciliary body, flows through the pupil, and drains through the trabecular meshwork and Schlemm's canal
- The trabecular pathway handles 80-90% of outflow; the uveoscleral pathway handles the remainder
- Impaired drainage at the trabecular meshwork leads to elevated IOP and glaucoma risk