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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 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. Some consider the pelvis a separate section, but there is no structure that separates the two areas.

Table of contents
1 Muscles of the Abdominal Wall
2 Organs of the Abdomen
3 Historical View of the (Human) Abdomen
4 Related topics
5 References

Muscles of the Abdominal Wall

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 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 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.

Organs of the Abdomen

The abdomen contains the liver with the gallbladder, stomach, pancreas, kidneys, spleen, small intestines with the appendix, colon and the urinary bladder. The kidneys, pancreas, and the major blood vessels 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.

Historical View of the (Human) Abdomen

In the mid-line a slight furrow extends from the ensiform cartilage above to the symphysis pubis below; this marks the linea alba in the abdominal wall, and about its middle point is the umbilicus or navel. On each side of it the broad recti muscles can be seen in muscular people. The outline of these muscles is interrupted by three or more transverse depressions indicating the lineae transversae in the recti; there is usually one about the ensiform cartilage, one at the umbilicus, and one between; sometimes a fourth is present below the umbilicus. The upper lateral limit of the abdomen is the subcostal margin formed by the cartilages of the false ribs (8, 9, 10) joining one another; the lower lateral limit is the anterior part of the crest of the ilium and Poupart's ligament running from the anterior superior spine of the ilium to the spine of the pubis; these lower limits are marked by definite grooves. Just above the pubic spine is the external abdominal ring, an opening in the muscular wall of the abdomen for the spermatic cord to emerge in the male. The most modern method of marking out the abdominal contents is to draw three horizontal and two vertical lines; the highest of the former is the transpyloric line of C. Addison, which is situated half-way between the suprasternal notch and the top of the symphysis pubis; it often cuts the pyloric opening of the stomach an inch to the right of the mid-line. The hilum of each kidney is a little below it, while its left end approximately touches the lower limit of the spleen. It corresponds to the first lumbar vertebra behind. The second line is the subcostal, drawn from the lowest point of the subcostal arch (tenth rib); it corresponds to the upper part of the third lumbar vertebra, and is an inch or so above the umbilicus; it indicates roughly the transverse colon, the lower ends of the kidneys, and the upper limit of the transverse (3rd) part of the duodenum. The third line is called the intertubercular, and runs across between the two rough tubercles, which can be felt on the outer lip of the crest of the ilium about two and a half inches (60 mm) from the anterior superior spine. This line corresponds to the body of the fifth lumbar vertebra, and passes through or just above the ileo-caecal valve where the small intestine joins the large. The two vertical or mid-Poupart lines are drawn from the point midway between the anterior superior spine and the pubic symphysis on each side vertically upward to the costal margin. The right one is the most valuable, as the ileo-caecal valve is situated where it cuts the intertubercular line, while the orifice of the vermiform appendix is an inch lower down. At its upper part it meets the transpyloric line at the lower margin of the ribs, usually the ninth, and here the gallbladder is situated. The left mid-Poupart line corresponds in its upper three-quarters to the inner edge of the descending colon. The right subcostal margin corresponds to the lower limit of the liver, while the right nipple is about half an inch above the upper limit of this viscus.

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