Adventist Youth Honors Answer Book/Nature/Cats/eyes
Cat's pupils are wide open in the dark, and contract to slits when exposed to the light. The back of the eye is coated with a reflecting surface. 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, the pupils of some (notably the ‘Big Cats’) appear more circular when dilated.