The pupil is a circular hole in the center of the iris and is about 3 mm to 4 mm in diameter in normal people in general light. The size of the pupil changes periodically with age over the course of a person’s life. The pupil diameter is smaller in young children within one year of age because the pupil opening muscle is not fully developed; it is larger in adolescents and adults; it is slightly larger in women than in men; it shrinks after age 40 in old age; it is larger in nearsighted eyes than in farsighted eyes; it is larger when awake than when asleep; and it shrinks when stimulated by bright light, emotional excitement, or severe pain. In short, under normal circumstances, the size of the pupil is in a constant dynamic change. So, why does the pupil change in size? One is the pupillary sphincter, which is arranged in a circular pattern around the pupil in the iris and shrinks the pupil when it contracts. The other is the pupillary sphincter, which is arranged radially in the iris and opens the pupil wide when it contracts. The two muscles are innervated by two types of nerves, and under normal conditions, they coordinate with each other to regulate the amount of light entering the eye by opening or closing the pupil in a variety of different situations. Clinical examination of the pupil is of great importance, whether in ophthalmology, internal medicine, neurology, or brain surgery. Doctors can determine the location and nature of the disease based on the size of the pupil and its response to light, which can be of great help in the diagnosis and prognosis of the disease.