The cornea is the transparent tissue layer that covers the front of the eye. It is about the size of a dime and is attached to the white portion of the eye called the sclera. A normal cornea is thin, clear, and does not contain blood vessels. One of its functions is to protect the more delicate parts of the eye, much like how the face of a watch protects its sensitive inner parts. The cornea is actually composed of five layers of tissue, all of which have very specific functions. The cells that make up the innermost layer of the cornea (endothelium) keep the cornea clear by regulating the amount of fluid that passes in and out of the cornea. These cells do not regenerate, so once they become damaged fluid can build up and vision can become cloudy. In these cases, only a corneal transplant can restore sight. In a cornea transplant, the diseased or damaged cornea is removed and replaced with donated corneal tissue.