The pupil is the black structure in the center of the iris of the eye. The pupil allows light to reach the retina of the eye.When the light hits the pupil, it changes size. Regulating the pupil’s size means regulating the amount of light entering the eye.
Although the function of the pupil is the same for every species, the shape is not. For example, cats have vertical slit pupils, goats have horizontally oriented pupils, and in humans, the pupil is round.
Let’s say your cat is in its normal surroundings, perfectly safe and simply lying in its cat tree. What shape and size pupils will your cat have? The answer is simple – the regular small, almond-shaped pupils.
A cat’s pupils being of different sizes is not considered normal. The most probable explanation for a cat having one pupil larger than the other is having a condition known as anisocoria.