The smooth transparent serous membrane (sac), consisting of mesothelium and a thin layer of irregular connective tissue, that lines the abdomen cavity and covers most of the viscera contained therein, and consists of an outer layer (peritoneal [or greater] sac) closely adherent to the walls of the abdomen and an inner layer (omental bursa [lesser sac] connected by the epiploic foramen) that folds to invest the viscera.