Pupil

In the eye, the pupil is the hole in the middle of the iris. It appears black because most of the light entering it is absorbed by the tissues inside the eye. In many animals (but few fish), the size of the pupil is controlled by involuntary contraction and dilation of the iris, in order to regulate the intensity of light entering the eye.

The shape of the pupil varies between species. Common shapes are circular or slit-shaped, although more convoluted shapes can be found in aquatic species. The reasons for the variation in shapes are complex; the shape is closely related to the optical characteristics of the lens, the shape and sensitivity of the retina, and the visual requirements of the species.

Slit-shaped pupils are found in species which are active in a wide range of light levels. In strong light, the pupil is small, but still allows light to be cast over a large part of the retina.

The orientation of the slit may be related to the direction of motions the eye is required to notice most sensitively (so a vertical pupil would increase the sensitivity of the eyes of a small cat to the horizontal scurrying of mice).

In snakes, slit-shaped pupils are associated with venomous species, while non-venomous snakes have round pupils.

When an eye is photographed with a flash, the iris cannot close the pupil fast enough and the blood-rich retina is illuminated, resulting in the red-eye effect.

Our understanding of how the eye works is still developing. The Lund Vision Group is one of the leading groups researching vision science.

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