Term: rod

Definition: Rods contain two segments-outer and inner-that are very distinct in osmicated preparations. ...All vertebrates including fish, have outer segments, although it is believed that they are absent in some birds; they are very abundant in nocturnal animals such as the larged horned owl, barn owl, rat and mouse. The inner segment is a bit longer and thicker than the outer segment. It is finely granular, stains lightly with carmine and the basic aniline dyes, and is unaffected by osmic acid. Rod perikarya (Fig. 190a) lie at various depths in the layer under consideration and contain a very small amout of cytoplasm, along with an ovoid nucleus that is smaller than that of cones.