The pupil is the space in the center of the iris that serves as a channel for light to enter the eye. The size of the pupil can be adjusted by the pupillary sphincter and pupil opening muscle, which in turn control the amount of light entering the pupil. The diameter of the normal adult pupil is generally in the range of 2.5-4mm. A narrow pupil, dilated pupil, or unequal bilateral pupils all have different pathological implications. Narrow pupils are most often seen in lesions such as cerebral bridge hemorrhage, while open pupils can also have dilated pupils on one or both sides. When the pupil is dilated on one side, it often indicates that the patient’s motor nerve is stimulated, suggesting the possibility of brain herniation, and should be given particular attention in clinical practice.