Abdomen (Lat., of undetermined etymology), the lower part of the body, included between the level of the diaphragm and that of the pelvis. The abdomen consists of its walls or boundaries, the cavity embraced by them, and the organs or viscera included therein. The walls are constituted below by the pelvis, a strong basin-shaped bone with wide flaring edges, upon the upper surface of which the weight of the abdominal organs is sustained; behind by a part of the spinal column and the strong muscles attached to its sides; above by the diaphragm, a vaulted muscular sheet, which forms the partition between the cavity of the abdomen and that of the chest; and in front by the abdomi-nal muscles and their integuments, extending from the lower part of the chest to the pelvis. In front and laterally, the abdominal walls are soft and flexible, being composed only of the skin, fatty tissue, fibrous membranes, and muscles; behind they are more solid and unyielding, owing to the bony framework of the spinal column, which here forms so large a part of their substance.

For convenience of anatomical examination and reference, the abdomen is divided externally into three nearly equal transverse bands or zones, an upper, middle, and lower; these zones being again divided into three nearly equal parts or "regions," namely, one middle and two lateral regions in each zone. In the upper zone the middle region is the epigastrium (Gr. Abdomen 1008 over, and Abdomen 1009 the stomach), because a portion of the stomach is situated immediately beneath it; the two lateral regions of the same zone being the right and left hypochondria (Abdomen 10010 under, and Abdomen 10011 a cartilage), because these two regions are beneath the cartilages of the lower ribs. In the middle zone, the median portion is the umbilical region, so called because it contains the umbilicus or navel; the two lateral portions are the right and left lumbar regions, or the loins. In the lowermost zone, the middle region forms the hypogastrium (and), and the two lateral portions the right and left iliac regions, which are occupied on each side by the ilium, or flaring portion of the pelvis. - The cavity of the abdomen is lined by a very extensive and delicate membrane, the peritoneum (Gr.to extend around), which is also reflected over the surfaces of the abdominal organs, as the covering of a chair or sofa may be reflected or extended over its cushion. In the case of those abdominal organs which remain fixed in their places, like the pancreas and the kidneys, the peritoneum simply passes over their anterior surfaces; but those which are movable, like the liver, stomach, and intestines, are more or less completely invested by it, some of them being attached to the posterior abdominal walls only by the double layer of peritoneum, returning upon itself after having covered their exterior. Thus these organs are covered, and the abdominal walls are lined, by opposite surfaces of the same continuous peritoneal membrane; and these surfaces are moistened by a minute quantity of serous fluid, which enables them to move gently to and fro upon each other, without causing friction or irritation of the parts. The organs contained in the abdomen are as follows: In the upper zone; the liver, stomach, spleen, pancreas, and the commencement of the small intestine; in the middle zone, the mass of the small intestine, with portions of the large intestine, the kidneys, and the suprarenal capsules; and in the lowermost zone, the remainder of the small and large intestines.

The very last portion of the large intestine occupies the deeper parts of the cavity of the pelvis, together with the urinary bladder and the uterine organs. - Owing to the flexible character of the abdominal Avails, much information may be obtained regarding the condition of the internal organs by external manual examination. If an organ be enlarged, indurated, or displaced, these changes may be detected by careful manipulation, and their increase or diminution may be determined from day to day. If one or more of them be inflamed, this condition is indicated by an unnatural tenderness on pressure; and the exact situation and character of the inflammation may often be fixed by observing whether the tenderness be superficial or deep-seated. Unnatural growths and tumors may be detected in the same way, and their origin ascertained in many cases with considerable approach to certainty. Penetrating wounds of the abdomen are very dangerous, because the contents of the stomach and intestines, if allowed to escape into the cavity of the peritoneum, produce an irritation and subsequent inflammation of the membrane; and this inflammation, spreading in every direction over the contiguous surfaces of the peritoneum, becomes so extensive and violent as almost invariably to produce fatal consequences.

Nevertheless, surgical operations in which the cavity of the abdomen is opened, but in which care is taken to prevent the escape or dissemination of irritating substances, have often been performed with a successful result. Sudden and powerful blows upon the abdomen, especially in the region of the epigastrium, are also sometimes fatal, even when none of the internal organs are lacerated, owing to the depressing influence of the shock upon the nervous system.