The cornea is the transparent part of the eye that covers the front portion of the eye. It contains no blood vessels, but it does contain many nerves and is very sensitive to pain or touch. It is nourished and provided with oxygen anteriorly by tears and is bathed posteriorly by intraocular fluid. It protects the inside of the eye from penetration by foreign bodies and is the first and most powerful element in the eye’s focusing system. The cornea's main function is to refract, or bend, light. The curvature of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which images appear elongated or distorted.