The pupil is a small round hole located in the center of the iris inside the eye of an animal or a human being, which is a channel for light to enter the eye, and the normal size of the pupil for an average adult is 2-5 millimeters. The pupil is a small round hole located in the center of the inner iris of an animal or human eye, which is the passageway for light to enter the eye, and the normal size of the pupil in general adults is 2-5 mm. The size of the pupil caliber and the iris smooth muscle is closely related to the pupil caliber narrowing or enlarging, to control the amount of light entering the pupil are dominated by this smooth muscle. The pupil is also the pathway for the anterior and posterior aqueous humor, and any obstruction to this will result in poor aqueous humor flow and elevated intraocular pressure, which is also the cause of secondary glaucoma. Tests commonly used to check for normal pupil function include measurement of pupil diameter and normalization of the direct and indirect light reflexes. Overall, the pupil is an indispensable structure of the eye. If a patient experiences eye discomfort, it is recommended that he or she consult a hospital in a timely manner to have the condition evaluated and treated according to the doctor’s instructions.