The pupil is the circular opening in the iris at the front of the eye. The size of the pupil determines the amount of light that enters the eye, and is controlled by the dilator and sphincter muscles of the iris. These muscles in turn react to changes in the light level received by the retina.

 

Pupil size is an important indicator of a person's state of health. Under fluorescent lighting of the kind that emergency physicians might use, normal pupil sizes range from 2.6 mm to 5 mm. In brighter lighting, normal pupil sizes range from 1.9 mm to 3.6 mm.

 

Several drugs affect the size of the pupil. For example, atropine eye-drops dilate the pupil and pilocarpine eye-drops constrict it.

 

Anisomelia is an inequality in size between the pupils of the two eyes, usually a difference of more than 1 mm in diameter.