The pericardium is a double-walled sac that surrounds the heart and the roots of the great vessels. It consists of two main layers: the fibrous pericardium and the serous pericardium. The serous pericardium further divides into two layers: the parietal layer, which lines the fibrous pericardium, and the visceral layer, which is also known as the epicardium and adheres directly to the heart.