The colored portion of the eye is called the iris. As with humans, dogs' iris colors vary. In the center of the iris is the black opening called the pupil. This opening can be made larger or smaller by muscles called ciliary bodies, that attach to the colored iris, causing it to expand or contract. The iris forms a diaphragm in front of the lens with a central sphincter-type opening called the pupil. It functions to regulate the amount of light entering the posterior segment of the eye.