Optic nerve: The optic nerve connects the eye to the brain. The optic nerve carries the impulses formed by the retina, the nerve layer that lines the back of the eye and senses light and creates impulses. These impulses are dispatched through the optic nerve to the brain, which interprets them as images. Using an ophthalmoscope, the head of the optic nerve can be easily seen. It can be viewed as the only visible part of the brain (or extension of it).
The optic nerve is the second cranial nerve. The cranial nerve 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.
The word "optic" comes from the Greek "optikos", pertaining to sight.
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.
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.