A type of light-sensitive receptor cell present in the retinas of vertebrates. Rods contain the pigment rhodopsin and are essential for vision in dim light. Each consists of a narrow cylindrical outer segment, which detects light; an inner segment containing mitochondria; a cell body containing the nucleus; and a terminal synaptic body, which transmits signals to an adjacent neuron. The rhodopsin molecules are organized within a stack of membranous discs in the outer segment. Rods are not evenly distributed on the retina, being absent in the fovea and occupying all of the retinal margin. See also dark adaptation. Compare cone.