Located around the anus, the opening of the rectum to the outside of the body. Peri- is a prefix borrowed from the Greek. It means "around or about." So pericardial is around the heart. Perinatal is around birth. And periumbilical is around the umbilicus (the belly button). Peri- is a useful prefix in anatomy and so is much employed in medicine. Pericardial fluid is fluid around the heart. Periaortic lymph nodes are lymph nodes around the aorta. A perianal abscess is an abscess (a local accumulation of pus) that forms next to the anus causing tender swelling in that area and pain on defecation.