The image below shows how this happens in the human eye. The difference with the cat's eye is that the pupil is in the shape of a slit when contracted. This actually provides greater control in restricting the amount of light entering the eye as the eyelid can close over the slit incrementally to leave a very small aperture. It is an added control.
![Eye dilate-thumb 300px](http://upload.wikimedia.org/wikipedia/commons/f/f0/Eye_dilate-thumb_300px.gif)
If the eyes are always dilated there is something wrong. A crude method to test for blindness is to shine a torch into a cat's eye to see if the iris contracts and the pupil constricts. Permanently dilated pupils may indicated advanced retinal disease.
Note: Image from Wikipedia Commons author: Greyson Orlando
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