The abdominal cavity is much more extensive than the impression gained from examination of the anterior abdominal wall. Much of it lies under cover of the lower ribs, for the domes of the diaphragm arch high above the costal margin. Hidden by the lower ribs are the liver and spleen, much of the stomach, and the upper poles of the kidneys and both suprarenals. The volume of the thoracic cavity is, correspondingly, much less than examination of the bony thorax would suggest. Furthermore, an appreciable amount of the abdominal cavity projects backwards into the pelvis, just in front of the buttocks. A perforating wound of the buttock can easily involve the pelvic cavity. The pelvic cavity accommodates not only its own pelvic organs (rectum, uterus, bladder, etc.), but also a goodly volume of intestine (sigmoid colon and ileum).
General topography of the abdomen
The alimentary canal and its two chief derivatives the liver and pancreas (and also the spleen) are developed in fetal mesenteries which later alter their disposition as a result of fusion of adjacent leaves of peritoneum. The liver and spleen remain invested in peritoneum, but the pancreas becomes retroperitoneal.
The alimentary canal is invested unevenly. Parts of it are suspended in the abdominal cavity by peritoneal folds (‘mesenteries’); other parts become plastered down to the posterior abdominal wall. The stomach is fixed at its two ends, but is elsewhere suspended by ‘mesenteries’. The duodenum is plastered down to the posterior abdom-inal wall, while the whole length of the small intestine swings free on its own mesentery. The ascending and descending colon are both adherent to the posterior abdominal wall, but between the colic flexures the transverse colon is mobile on its own mesentery, the transverse mesocolon. The sigmoid (pelvic) colon swings free on a mesentery, while the rectum is plastered by peritoneum to the hollow of the sacrum.
The suprarenals, kidneys and ureters lie behind the peritoneum. The aorta and inferior vena cava also lie behind the peritoneum, and intestinal vessels run through the mesenteries to reach the gut.
The transpyloric plane bisects the body between the jugular notch and the pubic symphysis. This level is approximately midway between the xiphisternum and the umbilicus, or about a hand's breadth below the xiphisternal joint (Fig. 5.1). It cuts each costal margin at the tip of the ninth costal cartilage, which is at the lateral border of the rectus abdominis (semilunar line); deep to this point on the right side lies the fundus of the gallbladder. The plane passes through the lower border of the first lumbar vertebra, where the spinal cord ends at the conus medullaris.
As its name implies, the plane usually passes through the pylorus, but the pylorus is suspended by the lesser and greater omenta, and is therefore relatively mobile. The plane passes along the head, neck and body of the pancreas, just above the attachment of the transverse mesocolon. The supracolic compartment (liver, spleen, fundus of stomach) lies above the plane, the infracolic compartment (small intestine, colon) below it. The superior mesenteric artery leaves the aorta, and the splenic vein joins the superior mesenteric vein to form the portal vein at this level. The hilum of each kidney lies at the plane, the right just below and the left just above it.