pupil

The pupil is the black circle in the middle of the eye. It is actually a hole in the middle of the iris that allows light to enter the back of the eye.  The size of the pupil is controlled by the iris muscle in response to ambient lighting.  In dark rooms, the pupil enlarges to allow more light to strike the retina. Conversely, in well-lit rooms the pupil constricts to protect the retina and to make the vision look crisper. While the pupil is normally round, there are some conditions that can make the pupil oddly shaped.  For example, inflammation of the iris (iritis) can make the pupil edge “stick” to the lens underneath,  causing the pupil shape to distort. This distortion is called iris synechiae.