retroflexion: Latin retro = backwards, and flexion = bent; hence, the position of being bent backwards, applied to the angulation of the body of the uterus on the cervix.
retroversion: Latin retro = backwards, and version = turned; hence, the position of being turned backwards, applied to the angulation of the cervix uteri on the vagina.
rhinencephalon: Greek rhinion = nostril, and enkephalos = brain; hence, the part of the brain concerned with smell (relatively large in lower animals).
rhombencephalon: Greek rhombos = rhomboid, and enkephalos = brain; hence, the hind-brain - the medulla oblongata, pons and cerebellum, which enclose the rhomboid fossa (the floor of the 4th ventricle).
rhomboid: Greek rhombus = a figure with 4 equal sides, not at right angles, and eidos = shape or form, hence, the shape of a rhombus.
rima: Latin = chink; hence, e.g., rima palpebrarum = the chink between the free edges of the eyelids.
risorius: Latin risor = scoffer; hence, muscle risorius is the facial muscle which expresses laughter by drawing the corner of the mouth laterally.
rostral: adjective, Latin rostrum = beak, implying nearness to the corpus callosum.
rostrum: Latin beak, which decorated the Roman orator's platform; hence, a platform or beak-like structure; adjective - rostral.
rotate: Latin rota = wheel; hence, to turn, and rotation, the act of turning.