Picture of Pericardial Sac
The pericardial sac is the tissue that lines the heart. It is comprised of multiple layers. The inner layers (visceral pericardium or epicardium) surround the heart and contain a cushion of fluid. This fluid serves as a lubricant and allows for movement of the heart within the chest. The outer layers (parietal pericardium) are thick and tough and attach to the sternum (breastbone) and the middle portion of the diaphragm.
Text Reference: National Heart, Lung, and Blood Institute: "What Is Pericarditis?"