Pericardium

The pericardium is a double-walled sac of fibrous tissue that contains the heart and the roots of the great vessels. The pericardium's outer coat is tough and thickened, loosely surrounds the heart, and is attached to the central part of the diaphragm and the back of the breastbone. Its inner coat (the visceral pericardium or epicardium) is double, with one layer closely adherent to the heart and the other lining the inner surface of the outer coat. The intervening space between these layers is filled with pericardial fluid. This small amount of fluid acts as a lubricant to allow normal heart movement within the chest.

The two layers to the pericardial sac are the fibrous pericardium and the serous pericardium. The serous pericardium, in turn, is divided into two layers, the parietal pericardium, which is fused to and inseparable from the fibrous pericardium, and the visceral pericardium, which is part of the epicardium.