The 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 separted 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. Some consider the pelvis a separate section, but there is no structure that separates the two areas.
The obliquus externus (external oblique) muscle is the outer most muscle covering the side of the abdomen. It is broad, flat and irregularly quadrilateral. It originates on the lower eight ribs, curves down and forward and its insertion is on the outer anterior crest of the ilium forward to the anterior superior spinous process.
The obliquus internus (internal or ascending oblique) muscle is triangularly shaped and is smaller and thinner then the external oblique muscle that overlays it. It originates from Poupart's ligament and the inner anteroir crest of the ilium. The lower two-thirds of it insert, in common with the transversalis, into the linea alba (a line of connective tissue running from the sternum to the pubis) and run from horizontal to nearly vertical. The upper third inserts into the lower six ribs.
The transversalis muscle is flat and triangular with its fibers running horizontally. It lays between the internal oblique and the peritoneum. It originates from, starting at the bottom, 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 from behind the rectus abdominis.
The rectus abdominis muscles are long and flat. They originate at the pubic bone, run up the abdomen on eather 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 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.
