Mydriasis iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is innervated by (signaled by nerves of) the parasympathetic nervous system, and the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the
What Is the What "we really hadn’t decided whether I was just helping Valentino write his own book, or if I was writing a book about him." Valentino points out that, "I thought I might want to write my own book, but I learned that I was not ready to do this. I was still taking classes in basic writing at Georgia Perimeter College." Dave Eggers discusses the difficulties in writing a book of this nature: "For a long while there, we continued doing interviews, and I gathered the material. But all along, I really didn’t know exactly what form it would finally take—whether