Optic nerve
From Memory Alpha, the free Star Trek reference.
In most humanoid species, the optic nerve connects the retina of the eye to the sensory thalamus, a structure within the brain that routes sensory information in the manner of a gateway. The optic nerve is part of the central nervous system. Unlike the peripheral nervous system, cells of the central nervous system do not regenerate, so damage to the optic nerve results in permanent blindness without medical intervention. The nerve is about 2 mm diameter where it emerges from the eye, increasing to roughly 4 mm at its terminus. Within the skull, the optic nerves of binocular species partially cross, so that each side of the brain processes infromation from both eyes; this is, in part, what makes binocular vision possible. (TOS: "Operation -- Annihilate!"; VOY: "Living Witness")
When the Cytherian probe sent a flash in order to establish contact with Reginald Barclay, there was enough energy in the flash to knock the Lieutenant unconscious. (TNG: "The Nth Degree")
See also: VISOR
