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In most humanoid species, the optic nerve connected the retina of the eye to the sensory thalamus, a structure within the brain that routed sensory information in the manner of a gateway.
The optic nerve was part of the central nervous system. Unlike the peripheral nervous system, cells of the central nervous system did not regenerate, so damage to the optic nerve resulted in permanent blindness without medical intervention. The nerve was about two millimeters in diameter where it emerged from the eye, increasing to roughly four millimeters at its terminus. Within the skull, the optic nerves of binocular species partially crossed, so that each side of the brain processed information from both eyes; this was, in part, what made binocular vision possible. (TOS: "Operation -- Annihilate!"; VOY: "Living Witness")