The pupil, not only responds to light and darkness and regulates the light entering the eye, but also affects the depth of focus and spherical aberration of the eye’s optical system. The pupil’s range of variation can be very large; when extremely constricted, the pupil of the human eye can be less than 1mm in diameter, while when extremely dilated, it can be greater than 9mm. The sphincter of the iris can shrink to 87% of its length, which is almost impossible for other smooth muscles or transverse muscles in the human body to achieve. The pupil itself will adjust its size according to the intensity of external light, strong light pupil becomes smaller, weak light, pupil becomes larger, this is the self-protection of the eye. We also see the pupil as a sensitive indicator of life functions. As you know, the pupil automatically narrows when the light is strong, and dilates when the light is low – this is called the pupil’s response to light. If the pupil’s response to light is dull or absent, it means that death is imminent. In comatose patients, the pupil will gradually dilate as the loss of consciousness increases, and complete dilatation is a sign of the end of life. 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. Under normal circumstances, the size of the pupil is in a constant state of dynamic change.