Final answer:
The pericardium, consisting of the fibrous pericardium and serous pericardium (including the parietal and visceral layers), is the protective layer that encloses the heart.
Step-by-step explanation:
The layer that encloses and protects the heart is called the pericardium, or pericardial sac. It is a membranous structure that consists of two distinct components: the rugged outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is composed of tough connective tissue that safeguards the heart and anchors it within the thorax. The serous pericardium further subdivides into two layers: the parietal pericardium that is fused to the fibrous pericardium, and the visceral pericardium or epicardium, which is attached to the heart itself. These layers of the pericardium are essential as they facilitate efficient heart function by allowing for substantial movement during pumping while reducing friction and maintaining the heart's position in the chest.