Pericardium: A loose-fitting sac where the heart lies. This sac is composed of an outer fibrous portion and an inner serous portion. The inner serous portion consists of two layers. 1. The parietal layer which lines the inside of the fibrous pericardium. 2. The visceral layer (epicardium) which adheres to the exterior surface of the heart. Between the parietal and visceral layers is a potential space, the pericardial space, which contains a few drops of pericardial fluid. The fluid reduces friction during movement of the heart.