Latin lacked a standard word for labia; two terms found in medical writers are "orae", "edges" or "shores", and "pinnacula", "little wings". The first recorded instance of the word "vulva" occurs in Varro's work on agriculture (1st century BC), where it refers to the membrane that surrounds a fetus. In the early Empire, "vulva" came into usage for "womb", the usual word for which had been "uterus" in the Republic, or sometimes more vaguely "venter" or "alvus", both words for "belly". "Vulva" seems originally to have referred to the womb of animals, but is "extremely common" in Pliny's "Natural History" for a human uterus. In the Imperial era, "vulva" can mean "female reproductive organs" collectively or vaguely, or sometimes refers to the vagina alone. Early Latin Bible translators used "vulva" as the correct and proper word for the womb. At some point during the Imperial era, "matrix" became the common word for "uterus", particularly in the gynecological writers of late antiquity, who also employ a specialized vocabulary for parts of the reproductive organs.