The pericardium (from the Greek περί, around and κάρδιον, heart) is a double-walled sac containing the heart and the roots of the great vessels.The pericardial sac has two layers, a serous layer and a fibrous layer.It encloses the pericardial cavity which contains pericardial fluid.n between the parietal and visceral pericardial layers there is a potential space called the pericardial cavity, which contains a supply of lubricating serous fluid known as the pericardial fluid. When the visceral layer comes into contact with heart (not the great vessels) it is known as the epicardium.