Muscle cells of neural origin are situated in stroma of the iris, which is formed by a loose connective tissue with a large number of pigmented cells. The muscle cells constitute the sphincter pupillae and dilator pupillae. These are fusiform cells #1, #2. The middle part of such a cell has an eminence with a nucleus; the cytoplasm contains actin and myosin filaments, general organelles, and small pigment inclusions #1, #2. Any muscle cell has its own innervation thereby ensuring a high speed of muscle contraction and a rapid change of the pupillary lumen size.