Cranial nerve, second

The second cranial nerve is the optic nerve, the nerve that connects the eye to the brain and carries the impulses formed by the retina — the nerve layer that lines the back of the eye, senses light and creates the impulses — to the brain which interprets them as images. The cranial nerves emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the vertebral column. There are twelve cranial nerves. In terms of its embryonic development, the optic nerve is a part of the central nervous system (CNS) rather than a peripheral nerve. The word "optic" comes from the Greek "optikos", pertaining to sight. Aside from the optic nerve, the eye has a number of other components. These include the cornea, iris, pupil, lens, retina, macula, and vitreous. The cornea is the clear front window of the eye that transmits and focuses light into the eye. The iris is the colored part of the eye that helps regulate the amount of light that enters the eye. The pupil is the dark aperture in the iris that determines how much light is let into the eye. The lens is the transparent structure inside the eye that focuses light rays onto the retina. The retina is, as mentioned, the nerve layer that lines the back of the eye, senses light and creates impulses that go through the optic nerve to the brain. The macula is a small area in the retina that contains special light-sensitive cells and allows us to see fine details clearly. The vitreous humor is a clear, jelly-like substance that fills the middle of the eye. In sum, the optic nerve is uniquely a part of both the eye and the brain. It is embryologically the brain's envoy to the eye and functionally the eye's envoy to the brain.