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The anatomy of the abdomen, the section of the torso in humans found between the chest and pelvic cavities, includes the muscles of the abdominal wall and the contents of the abdominal cavity within, along with the organs of the digestive, urinary, and immune systems. It may include other soft tissues found here, such as the fibrous membrane lining the abdominal cavity known as the peritoneum, the adipose tissue or fat stored under the skin and on top of the muscle, or the visceral fat distributed among the organs. The anatomy of the abdomen generally refers to the belly and its contents, and as such the spine and the muscles of the back located between the rib cage and the pelvis are not counted.
Bordered by the chest cavity above and the pelvic cavity below, the abdomen lies between the diaphragm, which fills the bottom of the rib cage and forms the ceiling of the abdominal cavity, and the pelvic brim. This is the name for the circular border of the rounded space known as the pelvic inlet that separates the upper pelvis from the lower pelvis. The anatomy of the abdomen includes the space above the pelvic brim. In other words, the bottommost portion of the abdominal cavity fills the space within the upper pelvis between the two curved ilium bones, while the space contained by the lower pelvis between the paired ischium and pubic bones is occupied by the reproductive organs.
Enveloped by the peritoneum, the abdominal cavity houses the primary organs of digestion as well as a few accessory organs indirectly related to digestion or to urinary or immune function. Digestive organs found here include those of the alimentary canal or gastrointestinal tract. These are the lowest segment of the esophagus, the stomach, the duodenum, jejunum, and ileum of the small intestine. Also found here are the cecum and appendix, which connect the small intestine to the large intestine. The ascending, transverse, descending, and sigmoid colons of the large intestine are in this cavity, as is the rectum.
Accessory organs of digestion included in the anatomy of the abdomen are the liver, pancreas, and gallbladder. Also found in the abdominal cavity are the spleen, which is considered part of the immune system. The kidneys, which regulate urinary function, are also found here.
The anatomy of the abdomen includes structures outside the abdominal cavity as well. Protecting the contents of the abdominal cavity as well as enabling forward bending, side bending, and twisting movements are the muscles of the abdominal wall. These include the rectus abdominis, external obliques, internal obliques, and transversus abdominis. The rectus abdominis, better known as the “six-pack” muscle, is the most superficial of the abdominal muscles and is what causes the trunk to curl forward.
Beneath that are the external and internal obliques, diagonally oriented muscles that produce the movements of trunk rotation, or twisting, and lateral flexion, or side-bending. The deepest muscle is the transversus abdominis. This wraps either side of the abdomen horizontally and works in opposition to the diaphragm to enable exhalation of air from the lungs during breathing.