Final answer:
The thin membrane lining the outside of the myocardium is the epicardium, which is part of the pericardium, the protective sac around the heart.
Step-by-step explanation:
The thin membrane lining the outside of the myocardium is known as the epicardium. The heart consists of three layers: the endocardium, which lines the inner chambers; the myocardium, which is the thick middle layer composed primarily of cardiac muscle cells; and the epicardium, which is the outer layer. The epicardium forms part of the pericardium, a protective sac that encloses the heart, consisting of two layers and housing a fluid-filled cavity to reduce friction during heartbeats.
The pericardium has two parts: the visceral pericardium or epicardium, and the parietal pericardium. The myocardium, the thickest layer of the heart, comprises cardiac muscle cells and is supported by a collagen framework. It contains both cardiomyocytes for muscle contraction and pacemaker cells for electrical impulse conduction, which coordinates the heart's contractions.