The pericardium is a double-walled sac that contains the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and the roots of the great vessels

Great vessels

Great vessels is a term used to refer collectively to the four large vessels that bring blood to and from the heart. These are:*Superior vena cava*Inferior vena cava*Pulmonary artery*AortaAll four pulmonary veins are considered apart of the great vessels...

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There are two layers to the pericardial sac: the outermost fibrous pericardium

Fibrous pericardium

The fibrous pericardium is the most superficial layer of the pericardium. It is made up of dense connective tissue, a loose connective tissue which acts to protect the heart, anchoring it to the surrounding walls, and preventing it from overfilling with blood...

 and the inner serous pericardium

Serous pericardium

The serous pericardium is deeper than the fibrous pericardium. It contains two layers, both of which function in lubricating the heart to prevent friction from occurring during heart activity:...

. The serous pericardium, in turn, is divided into two layers, the parietal pericardium, which is fused to and inseparable from the fibrous pericardium, and the visceral pericardium, which is part of the epicardium

Epicardium

Epicardium describes the outer layer of heart tissue . When considered as a part of the pericardium, it is the inner layer, or visceral pericardium, continuous with the serous layer....

. The epicardium is the layer immediately outside of the heart muscle proper (the myocardium).

The visceral layer extends to the beginning of the great vessels, becoming one with the parietal layer of the serous pericardium.