double-walled sac containing the heart and the roots of the great vessels
Epicardium (heart muscle) is one of the three main layers of the heart wall. It is the outermost layer of the heart, composed of connective tissue and fat. The connective tissue secretes a small amount of lubricating fluid into the pericardial cavity.
In addition to the connective tissue and fat, the epicardium is lined by, on its outer surface, simple epithelial cells.