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part of the secondary body cavity (coelom) of vertebrates; contains the viscera except for the kidneys, the heart and, in mammals, the lungs. In the adult stage of lower vertebrates, the canals uniting the abdominal cavity with the pericardial cavity are preserved. In many fish the abdominal cavity communicates with the external environment through abdominal pores. In cyclostomes and fish, the pericardial cavity is located in front of the abdominal cavity; in terrestrial vertebrates it moves back and opens into the abdominal cavity. In mammals, because of the formation of the diaphragm, the pericardial cavity and the anterior portion of the abdominal cavity (which contains the lungs) are separated from the rest of the abdominal cavity. In the majority of mammals, the abdominal cavity extends through the inguinal canal to the region of the scrotum. If, with age, the inguinal canal closes (for example, in carnivores and primates), the abdominal cavity becomes completely separated from the scrotal cavity. In the embryo the abdominal cavity is formed by the joining of paired coelomic cavities, which lie between the laminae of the lateral plate. The parietal peritoneum arises from the external (parietal) laminae of the lateral plate. The visceral peritoneum—that is, the serous membrane of the intestines and other internal organs—is formed from the visceral laminae (splanchnopleures); the mesentery is formed by the fusion of these layers. In cyclostomes, the abdominal cavity is lined with ciliated epithelium; in all other vertebrates it is lined with squamous epithelium.
In man the abdominal cavity, or stomach cavity, is bounded by the ventrolateral muscles of the abdomen, the lumbar vertebrae, the quadratus lumborum muscles, the diaphragm, and, below, the cavity of the pelvis major and pelvis minor. In the abdominal cavity are located the gastrointestinal tract (from the abdominal portion of the esophagus to the rectum), liver, pancreas, spleen, adrenal glands, and urogenital organs. Most of the unpaired abdominal organs are covered by the peritoneum, which marks the boundary of the peritoneal cavity (part of the abdominal cavity). The peritoneum in the region of the posterior wall of the abdominal cavity separates the peritoneal cavity from the retroperitoneal space, in which the paired abdominal organs (kidneys, adrenals, and ureters) and large blood vessels and nerve ganglia—surrounded by fatty tissue—are located.