Pericardium

Like the lungs, the heart and the roots of the great vessels are surrounded by a closed invaginated sac called the pericardium. Unlike the lungs, the parietal layer of the pericardium is reinforced by a tough fibrous outer coat.

The parietal pericardium has two layers: an outermost fibrous layer and an inner serous layer called the parietal serous pericardium. The two layers are fused and inseparable.

The visceral pericardium is also a serous layer called the visceral serous pericardium. This layer is part of the epicardium, which lies external to the heart muscle proper (the myocardium).

The space between the visceral serous pericardium and the parietal serous pericardium is called the pericardial space. It is filled with serous fluid, which protects the heart from any kind of external jerk or shock.