The iris is one of the more important components inside the eye, and in terms of appearance, the iris is where the black eyeball is located. Because the iris contains a lot of pigment, it will take on a darker color and will form the darker part of the eye. The iris is a disk of membrane-like tissue, so a round hole is formed in the middle, which is called the pupil. The size of the pupil is controlled by the iris, which has a pupillary sphincter. The contraction and relaxation of the pupillary sphincter controls the size of the pupil and changes the amount of light entering the eye, thus playing a regulatory role. If there is a disease of the iris in the eye, such as iritis, which is a type of uveitis, it can cause eye pain, tearing, and vision loss, and needs to be treated promptly so as to avoid further aggravation of the condition.