The double baglike fold of serous membrane which incloses the heart. [1913 Webster
" The inner layer is closely adherent to the outer surface of the heart, and is called the cardiac pericardium. The outer layer loosely incloses the heart and the adherent inner layer, and is called the parietal pericardium. At the base of the heart the two layers are continuous, and form a narrow closed cavity filled with fluid, in which the pulsations of the heart cause little friction."