Walsh & Hoyt: The Iris Muscles

The iris can be divided into four main parts from anterior to posterior: (a) the anterior border layer; (b) the stroma and sphincter muscle; (c) the anterior epithelium and dilator muscle; and (d) the posterior pigmented epithelium. Light and electron microscopic, histochemical, and angiographic techniques have been used to elucidate the features of these structures. The color of the iris is determined by its mesodermal and ectodermal components. In Caucasians, the stroma is relatively free of pigment at birth. The stroma absorbs the long wavelengths of light, allowing the shorter (blue) wavelengths to pass through to the pigmented epithelium where they are reflected back, causing the iris to appear blue. If the stroma is dense and contains numerous, heavily pigmented melanosomes, the delicate lacework of iris vessels is hidden by the pigment, and the surface of the iris looks brown and velvety.