The heart is a hollow muscular organ enclosed in a fibroelastic pericardial sac. In the wall of the heart, three distinct layers can be identified. The inner layer lining the chambers and covering the valves is the endocardium. The thick cardiac muscle of the heart is the myocardium. The epicardium is the thin layer of mesothelium and connective tissue that covers the surface of the heart as the visceral pericardium. The pericardial cavity, filled with fluid, separates the pericardium and epicardium.