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| Body cavities and spaces |
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The human body has three major body cavities. These are the craniospinal cavity, which contains the central nervous system (CNS) which includes the brain and the spinal cord; the thoracic cavity, which contains the lungs, heart and other viscera; and the abdominopelvic cavity, which contains the viscera of the abdomen and pelvis.
The abdominopelvic cavity is lined by a membrane called the peritoneum. The peritoneum can be found lining the walls of the cavity, in which case it is known as the parietal peritoneum. It can also be found covering most of the abdominal viscera; here it is known as visceral peritoneum. In some cases it will be found forming double layers, in which case it has different names, but we can use the generic term mesentery for these double layers.
The cavity bound by the peritoneum is known as the peritoneal cavity. The space outside the peritoneum, but still within the abdominal cavity, is known as the extraperitoneal space. The extraperitoneal space can be divided into three areas, all connected: an anterior, a lateral, and a posterior area. The anterior region is known as the preperitoneal space, the posterior region is known as the retroperitoneal space. All organs found in the retroperitoneal space are known as retroperitoneal organs.
As a very general rule, all retroperitoneal organs, some of which may be related to the peritoneum, do not have a mesentery and therefore are not mobile. This absence of mobility is what defines an organ as being retroperitoneal.
Also as a very general rule, most organs contained within the peritoneal cavity do have a mesentery and are therefore mobile. The presence of mobility defines an organ as intraperitoneal.
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