mydriasis /midrī″əsis/ [Gk, mydros, hot mass] , 1. dilation of the pupil of the eye caused by contraction of the dilator muscle of the iris, a muscular sheath that radiates outward like the spokes of a wheel from the center of the iris around the pupil. With a decrease in light or the pharmacological action of certain drugs, the dilator acts to pull the iris outward, enlarging the pupil. 2. an abnormal condition characterized by contraction of the dilator muscle, resulting in widely dilated pupils. Compare miosis. See also alternating mydriasis. −mydriatic, adj.
