A conical fibro-serous sac surrounding the heart and the roots of the great vessels (aorta; venae cavae; pulmonary artery). pericardium consists of two sacs: the outer fibrous pericardium and the inner serous pericardium. The latter consists of an outer parietal layer facing the fibrous pericardium, and an inner visceral layer (epicardium) resting next to the heart, and a pericardial cavity between these two layers.