Any of the muscles of the front and side walls of the abdominal cavity. Three flat layers—the external oblique, internal oblique, and transverse abdominis muscles—extend from each side of the spine between the lower ribs and the hipbone. The abdominal muscles attach to aponeuroses, connective tissue sheaths that merge toward the midline, sheathing the rectus abdominis muscle on each side of the midline. The abdominal muscles support and protect the internal organs and take part in exhaling, coughing, urinating, defecating, childbirth, and motion of the trunk, groin, and lower limbs.

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Largest hollow space of the body, between the diaphragm and the top of the pelvic cavity and surrounded by the spine and the abdominal muscles and others. It contains most of the alimentary canal, the liver and pancreas, the spleen, the kidneys, and the adrenal glands. It is lined by the peritoneum, a membrane covering the cavity's inside wall (parietal peritoneum) and each organ or structure in it (visceral peritoneum). Disorders include ascites (fluid in the peritoneal cavity) and peritonitis.

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