Double-layered sac. the innermost layer of this sac immediately surrounds the heart and is called the ____________________ pericardium (also called _______________). at the base of the heart, this inner layer folds back on itself to become the ________________ pericardium. between the layers of this sac is a potential space called the ______________ cavity and it is filled with _________ fluid. what is the function of this fluid?