The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of the eye's optical power.Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.Goldstein, E. Bruce. Sensation & Perception. 7th Edition. Canada: Thompson Wadsworth, 2007. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye's focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to "tune" the focus depending upon the object's distance. Medical terms related to the cornea often start with the prefix "kerat-".

Structure

The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air.

It borders with the sclera by the corneal limbus.

Layers

The human cornea, like that of other primates, has five layers. The corneas of cats, dogs, and other carnivores have only four.Merindano MD; Costa J; Canals M; Potau JM, and Ruano D. "A comparative study of Bowman's layer in some mammals: Relationships with other constituent corneal structures." European Journal of Anatomy. Volume 6, Number 3, December 2002. From the anterior to posterior they are:

1. Corneal epithelium: a thin epithelial multicellular tissue layer (stratified squamous epithelium) of fast-growing and easily-regenerated cells, kept moist with tears. Irregularity or edema of the corneal epithelium disrupts the smoothness of the air-tear film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity. It is continuous with the conjunctival epithelium is composed of about 6 layers of cells which are shed constantly on the exposed layer and are regenerated in the basal layer.
2. Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers, essentially a type of stroma. It is eight to 14 microns thick. This layer is absent in carnivores.
3. Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes. The corneal stroma consists of approximately 200 layers of type I collagen fibrils. 90% of the corneal thickness is composed of stroma. There are 2 theories of how transparency in the cornea comes about:
   1. The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils.(Maurice)
   2. The spacing of the neighbouring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek)