optic nerve, one of a pair of nerves that transmit visual impulses. The optic nerve is not a true cranial nerve but is rather an extension of the brain. It consists mainly of coarse myelinated fibers that arise in the retinal ganglionic layer, traverse the thalamus, and connect with the visual cortex. At the optic chiasm the fibers from the inner or nasal half of the retina cross to the optic tract of the opposite side. The remaining fibers from the temporal or outer half of each retina are uncrossed and pass to the visual cortex on the same side. The visual cortex functions in the perception of light, shade, and objects. The optic nerve fibers correspond to a tract of fibers within the brain. Also called nervus opticus, second cranial nerve.
