The abdomen is the part of the trunk inferior to the thorax (Fig. 4.21). Its musculomembranous walls surround a large cavity (the abdominal cavity), which is bounded superiorly by the diaphragm and inferiorly by the pelvic inlet.
The abdominal cavity may extend superiorly as high as the fourth intercostal space, and is continuous inferiorly with the pelvic cavity. It contains the peritoneal cavity and the abdominal viscera.