The pericardium consists of two layers, a visceral layer lined by mesothelial cells and a parietal or fibrous layer lined also by mesothelial cells but with attached fat and fibrous tissue. The mesothelial layer secretes a small amount of pericardial fluid, usually 50 ml of clear fluid which allows both surfaces to slide together during the cardiac cycle. The fibrous layer is usually 1 mm thick while the visceral layer is a transparent membrane on the surface of the heart. Pericardial diseases are relatively rare compared to the rest of the cardiac diseases, both valves and myocardium.