Which membrane covers the outside surface of the heart muscle?

The epicardium is the membrane that covers the outside surface of the heart muscle. It serves as a protective layer and is also part of the pericardium, which is the fibrous sac surrounding the heart. The epicardium plays a role in reducing friction between the heart and surrounding structures as the heart beats.

Understanding the structure of the heart provides clarity on the other options. The endocardium is a thin membrane that lines the interior of the heart chambers. The myocardium is the thick muscular layer responsible for the heart's contractions. The pericardium, while encompassing the heart, consists of two layers: the outer fibrous layer and the inner serous layer, of which the epicardium is the outer part. Thus, the epicardium is the correct answer as it is specifically the layer that directly covers the heart muscle itself.