Abdomen is a large body cavity between the thorax (chest) and the pelvic cavity. A strong wall of muscle, called the diaphragm, separates the abdomen from the thorax. But no structure separates the abdomen from the pelvic cavity. The abdominal organs include the stomach, liver, pancreas, intestines, kidneys, adrenal glands, and spleen. A thin membrane known as the peritoneum lines the entire abdominal cavity and covers most of the abdominal organs. Two large blood vessels, the aorta and vena cava, run along the spine and pass through the diaphragm and into the thorax.
The front wall of the abdomen consists of layers of sheetlike muscles attached to the ribs above and the pelvic bones below. These muscles hold in the abdominal organs and assist in bending and rotating the body trunk. Other muscles and the backbone form the rear wall of the abdomen.
The abdomen is the part of the body behind the thorax of insects, crustaceans, and certain other kinds of animals without backbones.
