The pericardium is a double-walled sac that surrounds the heart and roots of the great vessels. It has two layers - an outer fibrous pericardium and inner serous pericardium. The serous pericardium further divides into the parietal pericardium fused to the fibrous layer and the visceral pericardium lining the fibrous layer and covering the heart. Between these layers is the pericardial cavity lubricated with fluid. The pericardium supports and protects the heart, and its layers are supplied by nerves and blood vessels.