OPTIC NERVE

second cranial nerve that carries sensory nerve impulses from the retina to the visual cortex of the brain. In the eye, the nerve forms from the convergence of visual nerve fibres in the optic disk at the rear of the eyeball. The optic disk contains no receptor cells (cones) and is thus not capable of transmitting visual images; therefore it is called the blind spot. The nerve emerges from the eye at this point and passes along the underside of the brain to the optic chiasma, an X-shaped structure in which the optic nerves from each eye converge. Half of the nerve fibres from each eye continue to the visual cortex on the same side of the brain, and half cross over at the chiasma to join fibres from the opposite eye and continue to the visual cortex on that side, producing binocular vision. Although its major components are the visual fibres from the retina, the optic nerve also carries nervous impulses to and from the iris to regulate the pupillary reflex, a response to changing light conditions. Structurally, the optic nerve is a tract of the central nervous system rather than a true peripheral nerve.