The human abdomen (from the Latin word meaning "belly") is the part of the body between the pelvis and the thorax. The front of the abdomen is the abdominal cavity, which is separated from the thoracic cavity by the diaphragm. The lining of the abdomen is called the peritoneum, and the rear part of it is the retroperitoneum. The abdominal wall is the Skin, fat, muscle, and lining in the very front of the abdomen.

Muscles of the abdominal wall:

The obliquus externus (external oblique) muscle is the outermost muscle covering the side of the abdomen. It is broad, flat, and irregularly quadrilateral. It originates on the lower eight ribs, and then curves down and forward towards its insertion on the outer anterior crest of the ilium and (via the sheath of the rectus abdominus muscle) the midline linea alba. The obliquus internus (internal oblique) muscle is triangularly shaped and is smaller and thinner than the external oblique muscle that overlies it. It originates from Poupart's ligament/inguinal ligament and the inner anterior crest of the ilium. The lower two-thirds of it insert, in common with fibers of the external oblique and the underlying transversus abdominus, into the linea alba. The upper third inserts into the lower six ribs. The transversus abdominus muscle is flat and triangular, with its fibers running horizontally. It lies between the internal oblique and the underlying transversalis fascia. It originates from Poupart's ligament, the inner lip of the ilium, the lumbar fascia and the inner surface of the cartilages of the six lower ribs. It inserts into the linea alba behind the rectus abdominis. The rectus abdominis muscles are long and flat. They originate at the pubic bone, run up the abdomen on either side of the linea alba, and insert into the cartilages of the fifth, sixth, and seventh ribs. The muscle is crossed by three tendinous intersections called the linae transversae. The rectus abdominus is enclosed in a thick sheath formed, as described above, by fibers from each of the three muscles of the lateral abdominal wall. The pyramidalis muscle is small and triangular. It is located in the lower abdomen in front of the rectus abdominis. It originates at the pubic bone and is inserted into the linea alba half way up to the umbilicus (belly button).

Abdominal organs:

The abdomen contains the Liver with its attached GallBladder and the Small Intestine with its attached Appendix, plus the Stomach, Pancreas, Spleen, colon, and Urinary bladder. The kidneys, Pancreas, and major blood vessels such as the aorta and Inferior vena cava are also considered part of the abdomen, but are located in the part of the abdomen known as the retroperitoneum.