ABDOMINAL CAVITY

largest hollow space of the body. Its upper boundary is the diaphragm, a sheet of muscle and connective tissue that separates it from the chest cavity; its lower boundary is the upper plane of the pelvic cavity. Vertically it is enclosed by the vertebral column and the abdominal and other muscles. The abdominal cavity contains the greater part of the digestive tract, the liver and pancreas, the spleen, the kidneys, and the adrenal glands located above the kidneys. The abdominal cavity is lined by the peritoneum, a membrane that covers not only the inside wall of the cavity (parietal peritoneum) but also every organ or structure contained in it (visceral peritoneum). The space between the visceral and parietal peritoneum, the peritoneal cavity, is a potential space only, normally containing a small amount of serous fluid that permits free movement of the viscera inside the peritoneal cavity. This motion is particularly true of the gastrointestinal tract. The peritoneum, by connecting the visceral with the parietal portions, assists in the support and fixation of the abdominal organs. The diverse attachments of the peritoneum divide the abdominal cavity into several compartments, of which the compartment called the lesser sac is of particular importance; it is located behind the stomach and is connected to the rest of the peritoneal cavity, the greater sac, by the foramen (opening) of Winslow. Some of the viscera are attached to the abdominal walls by broad areas of the peritoneum, as is the pancreas. Others-e.g. the liver-are attached by folds of the peritoneum and ligaments, usually poorly supplied by blood vessels. The peritoneal ligaments are actually rather strong peritoneal folds, usually connecting viscera to viscera or viscera to the abdominal wall; their name usually derives from the structures connected by them (e.g., the gastrocolic ligament, connecting the stomach and the colon; the splenocolic ligament, connecting the spleen and the colon), or from their shape (e.g., round ligament, triangular ligament). The mesenteries are folds of peritoneum that are attached to the wall of the abdomen and enclosing viscera. They are richly supplied with vessels that carry blood to or from the organs they enfold. The three most important mesenteries are the mesentery for the small intestines; the transverse mesocolon, which attaches the transverse portion of the colon to the back wall of the abdomen; and the mesosigmoid, which enfolds the sigmoid portion of the colon. The omenta are folds of peritoneum enclosing nerves, blood vessels, lymph channels, and fatty and connective tissue. There are two omenta: the greater omentum hangs down from the transverse colon of the large intestine like an apron; the lesser omentum is much smaller and extends between the stomach and the liver. The presence of fluid in the peritoneal cavity (a condition known as ascites) is not noticeable until such a volume is attained as to distend the abdomen. The accumulation of fluid will produce pressure against the abdominal viscera and veins and also against the thoracic cavity by pressing upon the diaphragm. Treatment is directed toward alleviation of the cause. Decrease in portal vein pressure in many cases relieves the ascites that accompanies cirrhosis. Chylous ascites is best treated by closure of the leaking lymphatic vessel. Adequate treatment of heart failure will usually produce regression of the ascites that the heart failure has caused. Peritonitis, another disorder, is usually secondary to an inflammatory process elsewhere, which may come from an adjacent structure or organ; may be introduced from the outside by surgery or by injury; may come from organs in the abdomen; or may be borne by the bloodstream or the lymphatics. The most common origin of peritonitis is the gastrointestinal tract. Peritonitis may be acute or chronic, generalized or localized, and may be due to one agent or to a number of them. It is secondary to perforation of the intestines, for example. The severity of the reactions is related, at least in part, to the extent of the peritoneal contamination. In localized peritonitis, the surrounding structures, mainly the greater omentum, will enclose the infected area and temporarily control the infection. If no treatment is started, the infection may progress throughout the entire abdominal cavity. Often the period in which the peritonitis is localized is short, and the peritoneal inflammation becomes generalized with great rapidity. Control of the source of inflammation, either by surgical or by medical means, is followed by remission of all evidence of peritoneal inflammation or infection or by formation of localized abcesses inside of the peritoneal cavity. Antibiotic therapy has considerably decreased the incidence of the latter complication. When an abscess does develop, antibiotic therapy and adequate external drainage are necessary. The most frequent sites for development of abscesses are the spaces between the diaphragm and the pelvic cavity.