The cornea is a clear avascular tissue consisting of 5 layers (Fig 2-2):
The cornea covers one-sixth of the surface of the globe. It has a refractive index of 1.376 and an average radius of curvature of 7.8 mm. With a power of 43.25 diopters (D), the cornea produces most of the eye’s refractive power of 58.60 D. Oxygen from the air and from the eyelid vasculature dissolves in tears and is transmitted to the cornea via the tear film. The cornea derives its macromolecules and nutrients from the aqueous humor.
A, Histologic section showing the 5 layers of the cornea (thickness given within parentheses): epithelium (40–50 μm), Bowman layer (8–15 μm), stroma (470–500 μm), Descemet membrane (10–12 μm), and endothelium (4–6 μm). B, Anterior segment optical coherence tomography (AS-OCT) of the cornea. B = Bowman layer; D = Descemet membrane; En = endothelium; Ep = epithelium; S = stroma.
(Part A courtesy of George J. Harocopos, MD; part B courtesy of Vikram S. Brar, MD.)
Characteristics of the Central and Peripheral Cornea
In adults, the cornea measures about 12 mm in the horizontal meridian and about 11 mm in the vertical meridian. The central third of the cornea is nearly spherical and measures approximately 4 mm in diameter. Because the posterior surface of the cornea is more curved than the anterior surface, the central cornea is thinner (0.5 mm) than the peripheral cornea (1.0 mm). The cornea flattens in the periphery, with more extensive flattening nasally and superiorly than temporally and inferiorly. This topography is important in contact lens fitting. For additional discussion, see Chapter 2 in BCSC Section 8, External Disease and Cornea, and Chapter 4 in Section 3, Clinical Optics.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.