the ________ is the double-walled membranous sac that encloses the heart.
The pericardium (from the Greek ÏÎµÏÎ¯, around and ÎºÎ¬ÏÎ´Î¹Î¿Î½, heart) is a double-walled sac containing the heart and the roots of the great vessels. The pericardial sac has two layers, a serous layer and a fibrous layer. It encloses the pericardial cavity which contains pericardial fluid. The pericardium fixes the heart to the mediastinum, gives protection against infection, and provides the lubrication for the heart.