The Probert Encyclopaedia of Medicine

PUPIL

The pupil can be seen in the center of the iris of the eye where it appears to be black in colour. Around the pupil is a sphincter muscle that contracts according to the light conditions. If the eye is exposed to strong light, the pupil contracts, protecting the nerve cells at the back of the eye. In low light, the pupil enlarges to let in as much light as possible. The pupil also enlarges automatically when conversing with someone attractive. This automatic response is designed to make one appear more attractive to the other person, and thus improve the chances of a relationship.
Research Pupil