What is the X in X-mas?
pericardium per·i·car·di·um (pěr'ĭ-kär'dē-əm)
n. pl. per·i·car·di·a (-dē-ə)
The fibroserous sac enclosing the heart and the roots of the great vessels, consisting of two layers: the visceral layer or epicardium, immediately surrounding the heart; and the outer parietal layer, which forms the sac and is lined with a serous membrane. Also called heart sac, theca cordis.