Picture of Anus
Anus: The opening of the rectum to the outside of the body.
The word "anus" comes straight from the Latin. It referred to the same structure to the Romans. It also meant a "ring" in the sense of an encirclement which was entirely appropriate since the anus encircles the outlet of the bowel.
Image Source: MedicineNet, Inc.
Text: MedicineNet, Inc.