The optic nerve also known as cranial nerve II, or simply as CN II, is one of twelve cranial nerves but is also considered to be part of the central nervous system.
The optic nerve sends visual information from the retina to the brain. The brain forms a picture using the information so that you can see things.
Upside down images
The retina sees the image upside-down and the optic nerve sends it to the brain so it turns it back into normal.