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The optic nerve is the second cranial nerve, embryologically and structurally it ca be considered as a tract of the central nervous system rather than a true peripheral nerve. The optic nerve carries the nerve impulses from the retina to the visual cortex of the brain. The nerve fibres form the optic disc, the blind spot, as they leave the eye ball, at this point they share the opening with the vessels responsible for the blood supply to the inner eye, the central retinal artery and the central retinal vein. These blood vessels leave shortly after the opening and the optic nerves pass under the brain and converge at the optic chiasm. At this X shaped structure there is a mingling of the fibres from the two optic nerves, half the fibres from each nerve cross over to continue to the visual cortex. The way the fibres are arranged allows those carrying impulses from the right half of both retina to continue to the right side of the cortex and those carrying the impulses from the left portion of both retinae pass to the left.
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