Final answer:
The pericardium is the tough membrane that encloses and protects the heart. It consists of the outer fibrous pericardium and inner serous pericardium, which includes the parietal and visceral (epicardium) layers.
Step-by-step explanation:
The tough membrane that encloses the heart is known as the pericardium or pericardial sac. This membrane surrounds both the heart and the 'roots' of the major blood vessels. The pericardium features two main sublayers: the outer fibrous pericardium, made of dense connective tissue that protects the heart and secures its position in the thorax, and the serous pericardium. The serous pericardium itself includes two further layers, with the parietal pericardium being the part fused to the fibrous pericardium, and the visceral pericardium, or epicardium, directly covering the heart muscle. The inner wall of the heart is covered by the endocardium, and the heart muscle layer, referred to as the myocardium, makes up the bulk of the heart wall. A key function of the pericardium is to allow the heart enough room for its vigorous pumping actions while keeping it in place, thereby minimizing friction with surrounding structures.