The abdomen (abdominopelvic cavity) is the largest cavity in the body. It is of an oval shape, the extremities of the oval being directed upward and downward.
The upper extremity is formed by the diaphragm which extends as a dome over the abdomen, so that the cavity extends high into the bony thorax, reaching on the right side, in the mammary line, to the upper border of the fifth rib; on the left side it falls below this level by about 2.5 cm.
The lower extremity is formed by the structures which clothe the inner surface of the bony pelvis, principally the Levator ani and Coccygeus on either side. These muscles are sometimes termed the diaphragm of the pelvis.
The abdominopelvic cavity is wider above than below, and measures more in the vertical than in the transverse diameter. In order to facilitate description, it is artificially divided into two parts: an upper and larger part, the abdomen proper (abdominale cavity); and a lower and smaller part, the pelvis (pelvic cavity). These two cavities are not separated from each other, but the limit between them is marked by the superior aperture of the lesser pelvis.
The abdomen proper differs from the other great cavities of the body in being bounded for the most part by muscles and fasciæ, so that it can vary in capacity and shape according to the condition of the viscera which it contains; but, in addition to this, the abdomen varies in form and extent with age and sex. In the adult male, with moderate distension of the viscera, it is oval in shape, but at the same time flattened from before backward. In the adult female, with a fully developed pelvis, it is ovoid with the narrower pole upward, and in young children it is also ovoid but with the narrower pole downward.
Boundaries.—It is bounded in front and at the sides by the abdominal muscles and the Iliacus muscles; behind by the vertebral column and the Psoas and Quadratus lumborum muscles; above by the diaphragm; below by the plane of the superior aperture of the lesser pelvis. The muscles forming the boundaries of the cavity are lined upon their inner surfaces by a layer of fascia.
The abdomen contains the greater part of the digestive tube; some of the accessory organs to digestion, viz., the liver and pancreas; the spleen, the kidneys, and the suprarenal glands. Most of these structures, as well as the wall of the cavity in which they are contained, are more or less covered by an extensive and complicated serous membrane, the peritoneum.