Cornea

The tough, clear membrane that covers the front of the eye and protects the inner parts of the eye is called the cornea (See Anatomy of the Eye). It serves as the outer lens of the eye and provides approximately 70 percent to 80 percent of the eye's refractive power. Fine-tuning is provided by the natural crystalline lens that lies behind the pupil. Together these two lenses focus light on the retina at the back of the eye, which sends electrical signals to the brain enabling a person to see. The shape of the cornea and its general condition determine, to a large degree, the visual powers of the eye.

The cornea comprises five layers. The outermost layer is called the epithelium. It is a covering like the skin, only it's transparent. The majority of nerve endings of the cornea are in this layer, which is why the cornea is sensitive to touch. If damaged, the epithelium grows back in a few days without scarring. Tear film lubricates the epithelium, and inadequate tear film results in dry eye.

Just below the epithelium is the Bowman's membrane. Composed of strong collagen fibers, this smooth layer helps the cornea maintain its shape. If the Bowman's membrane is damaged however, scarring will normally occur.

The third and thickest layer of the cornea is the stroma, a layer of neatly compacted fibers that provide the strength for the eye to withstand trauma. Regular spacing of these fibers is critical to the clarity of the cornea. Infection, injury, or disease to the stroma will heal. However, scar tissue may cause a disruption in the collagen pattern, resulting in decreased vision.

Decemets' membrane is located directly behind the stroma. It serves as a barrier to infectious organisms while allowing water and nutrients to pass through.

The fifth and innermost layer of the cornea is the endothelium, which is a delicate, one-celled-thick layer that acts as a fluid regulator for the stroma. It maintains the cornea in a relative state of dehydration, thereby keeping the collagen fibers neatly compacted and allowing the transmission of light.

Although the cornea contains the highest concentration of nerve fibers of any structure in the human body, it contains no blood vessels. That is one reason why the cornea remains clear. The cornea receives nutrition from the fluid interior of the eye and from the outer tear film surface.