The basic function of the vertebrate iris, which is aptly compared to the aperture diaphragm of a camera, is control of the intensity of light reaching the retina. The pupillary aperture is controlled by two sets of antagonistically acting muscles in the iris: the circular muscle of the sphincter pupillae (iris sphincter), which causes narrowing of the pupil (miosis), and the radially arranged muscles of the dilator pupillae (iris dilator), which causes widening of the pupil (mydriasis). In most groups the iris sphincter and dilator are composed of smooth muscle, but in reptiles and birds the iris muscles are, at least mainly, striated. In some groups (fish, amphibians and reptiles) the iris sphincter contains a light-sensitive pigment which causes a direct constriction of the pupil in response to incident light, without involvement of nervous mechanisms (Brown-Séquard 1847, Steinach 1890, 1892, Guth 1901). The pupillary diameter is controlled mainly by the sphincter and, in some species of teleosts, amphibians and reptiles dilator muscles may be absent (see below). It is of interest to note that the muscles of the iris are derived from the embryonic retina (light-sensitive pigments!) and are thus of ectodermal origin. In spite of this unorthodox origin, the muscles of the iris show both structural and functional similarities with other muscle tissue (Romer 1962).
KeywordsPupillary Diameter Oculomotor Nerve Sympathetic Chain Autonomic Innervation Ciliary Ganglion
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