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Wednesday, November 15, 2006

All About A Cat's Eyes

Unlike human eyes, which work best in day light, the cat’s eyes
must function well in extremely low light condition and as such
are well suited to an animal that is predominantly nocturnal and
crepuscular in activity. In darkness, cats eyes are able to
function in approximately one-sixth of the light needed for human
vision. However they must also be able to function well in
daylight – so just how is this achieved.

In low light levels the cats pupil must be able to open as wide
as possible, but also be able to contract to very small size to
protect the sensitive retina in bright sunlight. In human eyes,
this size variation of the pupil is controlled by a circular
ciliary muscle, but this limits the amount of size variation.


In cats however, the same process is controlled by two, shutter-like
ciliary muscles, which gives the cat it’s characteristic
slit-like pupil in bright light conditions. All cats pupils are
therefore elliptical, however some, notable the ‘Big Cats’,
appear more circular when dilated.

The size of the cats eye is relatively larger than those found in
human, this enables a larger pupil and therefore more light to
enter the eye. Generally, the lens is more curved enabling
sharper focusing even at the edges of the lens. The size of the
anterior chamber and the curvature of the cornea is also greater,
which helps more light to be refracted onto the light-sensitive
retina.

Another feature, which enabled the amount of light hitting the
retina to be increased, is the tapetum lucidum. This is
positioned at the back of the eye, behind the retina and acts
like a mirror, reflecting light back onto the light sensor cells
in the retina. This gives the cats eyes the characteristic
night-time glow when they are caught in a beam of light.

There are two distinct types of light receptor cell on the retina
– Cones, which are sensitive to high levels of light, used in
colour vision and Rods, sensitive in low light conditions. In
cats, there is a greater concentration of Rods, aiding their
night-time vision. As in humans, there is a greater concentration
of receptor cells at the center of the eye, leading to the optic
nerve. In cats, however, these a concentrated along a broader,
horizontal band. This gives the cat far more sensitivity to
movement along the horizontal axis and they are therefore more
able to detect prey movement along the ground at greater
distances.


Eye color is genetically related to coat color.

Pointed cats always have blue eyes.
White cats, and cats with a lot of white markings, can have:
blue eyes
green, gold, or copper eyes
or "odd-eyes" (one blue eye and one green or gold eye)!
Other cats can only have green, gold, or copper eyes, not blue eyes. The most common eye colors are in the middle of the eye color spectrum (greenish-yellow to gold). The colors at the ends of the eye color spectrum (deep green or brilliant copper) are usually seen only in purebreds who have been selectively bred for extreme eye color, but they may sometimes appear in non-purebreds.

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