The pupil is the opening in the center of the iris (the structure that gives our eyes their color). The function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight.
Together, the iris and pupil control how much light enters the eye.
Using the analogy of a camera, the pupil is the aperture of the eye and the iris is the diaphragm that controls the size of the aperture.
Generally, normal pupil size in adults ranges from 2 to 4 millimeters (mm) in diameter in bright light to 4 to 8 mm in the dark.
In addition to being affected by light, both pupils normally constrict when you focus on a near object. This is called the accommodative pupillary response.
The size of the human pupil may also vary as a result of age, disease, trauma, or other abnormalities within the visual system, including dysfunction of the pathways controlling pupillary movement. Thus, careful evaluation of the pupils is an important part of both eye and neurologic exams.
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