Merriam-Webster Collegiate Dictionary - retina

 noun  (plural ~s or ~e)  Etymology: Middle English rethina, from Medieval Latin ~, probably from Latin rete net  Date: 14th century the sensory membrane that lines the eye, is composed of several layers including one containing the rods and cones, and functions as the immediate instrument of vision by receiving the image formed by the lens and converting it into chemical and nervous signals which reach the brain by way of the optic nerve — see eye illustration