The eye is the organ of vision, and the optic nerve CN II grows out from the brain and enters the retina. The muscles that move the eye are attached to the outer surface of the eye. Of all our senses, vision is the most important: we perceive the world mostly through our eyes. Even though light intensity varies by a factor of ten million between the brightest snowy day and a starlit night, our eyes and visual system adapt to these intensity changes. We can discriminate between thousands of hues and shades of color. Our eyes are set in our heads in such a way that each eye sees almost the same visual field, making depth perception possible. In the visual system, the primary, secondary, and tertiary neurons are in the retina and are all part of the central nervous system. The right field of vision projects to the left cerebral hemisphere; the left field of vision projects to the right cerebral hemisphere.