The retina is brain tissue that is part of the central
nervous system. We tend to see the world the way we are wired to
see it, although the brain is marvelously flexible in regard to
perception. If you optically invert your view, the brain puts it
right side up again. If you take away sight, it can use sound to
see your surroundings.
See also in The book of science:
Readings on wikipedia: