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 a Pleasure Stakes What a Pleasure Stakes The What a Pleasure Stakes is an American flat Thoroughbred horse race for two-year-olds held annually at Calder Race Course in Miami Gardens, Florida. It is currently an ungraded stakes race run over a distance of 8.5 furlongs on dirt. The race has traditionally been held during the Tropical at Calder meet, from late October through early January. This meet normally has some of the strongest stakes programs of Calder’s eight-month season. Due to permit disputes with Hialeah Park Race Track, the race was pushed back in 1987 and 1991 into the following year. Both of