A circular smooth muscle cell of the iris, innervated by the ciliary nerves (parasympathetic), and acting to contract the pupil. This muscle cell derives from neuroectoderm. This smooth muscle cell results from transformation of epithelial cells to smooth muscle cells. [ https://www.worldcat.org/search?q=bn%3A0721662544 https://www.worldcat.org/search?q=bn%3A0721694128 ]