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The retina is the thin layer of cells that lines the back of the eyeball in humans and in many animals. It is made mainly of neurons and its role is to sense the light which gets into the eye and send information about it to the brain.
The information about light is for two different purposes, and it takes two different paths. Information about light which travels through the optic nerve is for sight. Information about light which travels through the retino-hypothamalic tract is for adjusting circadian rhythms to the 24-hour day.