Human eye. Close-up of a blue eye. The pupil (black) allows light to enter the eye. A ring of coloured muscles, the iris (blue), controls the size of the pupil. The white of the eye, the sclera, is the tough outer wall that keeps the eyeball spherical. At the front of the eye the sclera is replaced by a circular transparent area, the cornea, which covers the iris and pupil. Blood vessels in the choroid layer beneath the sclera and cornea supply the eyeball with nutrients and oxygen. The visible surface of the eye is covered by the conjunctiva, a transparent membrane. Eyelashes surrounding the eye are modified hairs that trap airborne debris.