The outer layer of the eyeball is called sclera. It is colorless, opaque, hard and elastic. It is composed of cartilage and connective tissue. At the front of the eye, the sclera changes to form a very thin and transparent cornea. The cornea is called the window of the eye. There is no blood supply to the cornea. It receives nutrients from the aqueous humor. Outside the cornea is a thin and transparent conjunctiva. The part behind the sclera through which the optic nerve enters is called the lamina cribosa.