Abdo'men, in man, the lower cavity of the trunk, separated from the upper cavity, or thorax, by the diaphragm and bounded below by the bones of the pelvis. It contains the intestines, liver, stomach, spleen, pancreas, kidneys and other organs. A serous membrane, called the peritoneum, lines the cavity and is reflected from it in such a way as to enclose the contents, giving them the necessary freedom of movement and at the same time keeping them in their proper position. This membrane is the seat of the disease peritonitis. The chief organs of the abdomen and chest are shown in the plate. See plate facing next page.