Protective membrane that protects the heart?

The membrane that protects the heart is called the pericardium. It is a double-layered sac that surrounds the heart and the roots of the major blood vessels. The outer layer of the pericardium is tough and fibrous, while the inner layer is smooth and serous. The pericardial cavity between the two layers contains a small amount of serous fluid that reduces friction between the heart and the pericardium.