Thursday 6 October 2011

Are cats eyes dilated always?

The answer is a positive, No. Dilated eyes are eyes in which the opening in the center of the eye, the pupil, is enlarged letting in more light. When the iris, the pigmented muscle surrounding the pupil, contracts the pupil closes and lets in less light.  This occurs when the ambient light is bright. When it is dark a cat's eyes should be dilated.

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

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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