The tiny retina of the eye is packed with more sensory nerve cells than any other organ in the body. When light enters the eye, it passes through the lens and is focused onto the retina. Comprised of light-sensitive nerve tissue located at the rear of the eyeball, the retina sends messages directly to the brain by way of the optic nerve. This specialized eye tissue contains two different kinds of light receptor cells, the rods and the cones. Each cell type is comprised of chemicals that react to various wavelengths of light. Cones cells provide the visual centers of the brain with details that are used in reading and color perception and are especially prominent in animals that are active during the day. Rod cells are highly sensitive to reduced light intensities, and not surprisingly, dominate the retina of night dwelling or nocturnal animals. The eyes of some nocturnal creatures including bats, some snakes, and lizards have no cone cells at all, while the retina of others creatures contain just a few.

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