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The pupil is the part of the eye that allows the passage of light into the vitreous humor.  It is found at the center of the iris and is merely a clear part of the lens of the eye.  However, it is often referred to instead of the iris when discussing a patient's symptoms because the size of the pupil is often easier to discern than the iris that surrounds it which is actually doing the reacting.

The size of the pupil is generally related to the amount of light in the environment.  In dim or dark conditions, the pupil is about six times larger than it is in very bright conditions.  In a normal patient, the pupil reacts quickly to light, even the light from a physician's flashlight in a darkened room. 

Pupil response is a common test of brain function in a patient as the pupil will react normally in most unconscious or sleeping patients.  Failure of the pupil to react is a sign of brain damage and is one of the key signs in determining if a patient is in a coma.

Pupils will also react to the use of certain drugs.  For example, most stimulants will cause the pupils to become fully dilated, even in bright light, often leading to photophobia.