rod [AS, rodd] , 1. a straight cylindric structure. 2. one of the tiny cylindric elements arranged perpendicular to the surface of the retina. Rods contain the chemical rhodopsin, which adapts the eye to detect low-intensity light and gives the rods a purple color. Each rod is 40 to 60 μm in length and about 2 μm thick and consists of a slender reactive outer segment and an inner granular segment. When bright light strikes a rod, rhodopsin rapidly breaks down; it reforms gradually in low-intensity light. Compare cone. See also iodopsin, Jacob x membrane, rhodopsin.
