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Abdomen, the lower of the two cavities into which the trunk of the human body is divided by the diaphragm. Below, the abdominal cavity is continuous with that of the pelvis (q.v.), the boundary between the two being known as the pelvic brim. For convenience of reference, the abdomen is described as consisting of three zones, an upper, middle, and lower, each zone being again divided into three parts, thus forming nine regions in all. The epigastrium (A) is the middle region of the upper zone, having on either side the right (B) and left (C) hypochondriac regions. In the middle zone is the umbilical (D), bounded on either side by the right (E) and left (F) lumbar regions; while the lowest zone presents laterally the right (H) and left (I) inguinal regions, including between them the hypogastrium(G). The liver lies mainly in the right hypochondrium but extends into the epigastrium; the spleen is found in the left hypochondrium; the stomach occupies the epigastrium and part of the left hypochondrium; and the pancreas is placed transversely across the superior zone, lying mainly in the epigastric or middle region, but extending into the lateral regions on either hand. The two kidneys are situate in the right and left lumbar regions respectively. The cascum or first part of the large intestine lies in the right inguinal region, and the succeeding parts are the ascending colon, which passes upwards through the right lumbar region, the transverse colon, which runs transversely across the umbilical, the descending colon, which passes through the left lumbar, and the sigmoid flexure which occupies the left inguinal region; the terminal portion, the rectum, being found in the pelvis. The convolutions of the small intestine occupy mainly the umbilical and hypogastric, but extend into the right and left lumbar regions. The abdomen is lined throughout by a serous membrane, the peritoneum (q.v.), which is reflected over the several viscera, and serves to maintain them in position. One of the chief surgical advances of modern times has been made in connection with the abdomen. The operation of opening the abdominal cavity is now not infrequently undertaken for the relief of certain diseased conditions, and a large number of cases have now been conducted to a successful issue, which in former days would have been regarded as of too desperate a nature to admit of alleviation or cure.

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