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The Nocturnal Eye

What appears as pitch black to a human may be dim light to a nocturnal animal. The reason lies in the structure of the eye itself.
Diagram of the nocturnal eye


Pupils
Nocturnal animals tend to have proportionally bigger eyes than humans do. They also tend to have pupils that open more widely in low light. So, at the outset, nocturnal eyes gather more light than human eyes do.

Rods and cones
After the light passes through the pupil, it is focused by the lens onto the retina, which is connected to the brain by the optic nerve. The retina is an extremely complex structure. It's made up of at least 10 distinguishable layers, and is packed with more sensory nerve cells than anywhere else in the body.

The retina is home to two different kinds of light receptor cells—rods and cones. (Both are named after their relative shapes.) Cones work in bright light and register detail, while rods work in low light, detecting motion and basic visual information. It is the rods that become highly specialized in nocturnal animals. In fact, many bats, nocturnal snakes and lizards have no cones at all, while other nocturnal animals have just a few.

Tapetum
Many nocturnal eyes are equipped with a feature designed to amplify the amount of light that reaches the retina. Called a tapetum, this mirror-like membrane reflects light that has already passed through the retina back through the retina a second time, giving the light another chance to strike the light-sensitive rods. Whatever light is not absorbed on this return trip passes out of the eye the same way it came in—through the pupil. The presence of the tapetum can be observed at night when a pair of glowing eyes reflects back a flashlight or some other light source. (Interestingly, different animals have different color tapeta, a fact that can aid in nighttime animal identification.)

Diagram of different forms of pupils: circular vs. slit

Circular vs. slit pupils
One consequence of having extremely light sensitive eyes, is that they must be adequately protected during the day. Some animals accomplish this with a retractable eye flap. Others rely on their pupils.

The circular pupil, because of the way the muscle bunches as it contracts, is the least efficient at closing rapidly and completely. A slit pupil, with two sides that can close like a sliding door, is far better at this task, which is why so many nocturnal eyes have slit pupils. These apertures can be vertical, horizontal, or diagonal.

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