The circle circumference sphincter constricting muscle is the opposing muscle of the circle-radius dilator muscle. The sphincter muscle has a parasympathetic innervation, and the dilator has a sympathetic innervation. It is typically defined as the region where the sphincter muscle and dilator muscle overlap. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. The well-developed dilator muscle gives the animals a great range in pupil dilation. The "dilator pupillae" dilates the pupil; its action is antagonistic to the "sphincter pupillae". In Horner's syndrome, the sympathetic innervation to the pupillary dilator muscle is lost. This anterior surface projects as the dilator muscles. It inactivates the dilator muscle and thereby produces miosis. The hygroscopic nature of the polymer causes the dilator to absorb fluid and expand. They are removed by grasping the strings of the dilator and applying gentle traction. It plays a role in the control of the iris dilator muscle. The dilator naris helps to form the upper ridge of the philtrum. Both pumps are powered by substantial dilator muscles in the head. Rectovaginal fistulae have also been linked to dilator use.