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The cornea is the clear window at the front of the eye. It is a curved membrane about 12mm in diameter consisting of regularly arranged collagen fibres. The cornea has no direct blood supply but does contain a large number of nerve fibres making it very sensitive to touch or pain. As well as preventing injury to the other structures in the eye the cornea provides some of the refractive power of the eye.
The sclera forms the "white" of the eye-ball. Like the cornea, the sclera is composed of collagen fibres, to form a hard, protective layer, however, unlike the cornea the collagen fibre are not arranged in a regular way. The sclera is pierced by numerous nerves and blood vessels and also provides the anchor point for the six extra-ocular muscles. The largest opening in the sclera, the posterior scleral foramen, is caused by the optic nerve.
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