Abdomen -- (L., possibly from abdere to hide) that portion of the body which lies between the thorax and the pelvis; called also belly and venter. It contains a cavity (abdominal cavity) separated by the diaphragm from the thoracic cavity, above and by the plane of the pelvic inlet from the pelvic cavity below, and lined with a serous membrane, the peritoneum. This cavity contains the abdominal viscera and is enclosed by a wall (abdominal wall or parietes) formed by the abdominal muscles, vertebral column, and the ilia. It is divided into nine regions by four imaginary lines projected onto the anterior wall, of which two pass horizontally around the body (the upper at the level of the cartilages of the ninth ribs, the lower at the tops of the crests of the ilia), and two extend vertically on each side of the body from the cartilage of the eighth rib to the centre of the inguinal ligament, as in A below. The regions are : three upper right hypochondriac, epigastric, left hypochondriac; three middle right lateral, umbilical, left lateral; and three lower right inguinal, pubic, left inguinal).