 Constriction of a circular pupil (by a ring-shaped muscle) is less complete than closure of a slit pupil, which uses two additional muscles that laterally compress the pupil.For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans.However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5 mm in the tarsier) and the rectangular pupils of many ungulates which do not close to a narrow slit in bright light.