Which muscle layer forms the outer surface of the heart?

The epicardium is indeed the muscle layer that forms the outer surface of the heart. It is a thin layer of tissue that adheres to the heart itself and is part of the larger structure called the pericardium. The epicardium contains blood vessels, nerves, and fat and plays a vital role in protecting the heart and contributing to its structure.

This layer is important because it acts as a protective barrier for the underlying myocardium, which is the heart muscle responsible for contractions and pumping blood throughout the body. Understanding the layers of the heart is crucial for recognizing how the heart functions and how various conditions can affect it, making the epicardium's role noteworthy in cardiovascular health and disease.

Other choices refer to different layers or structures related to the heart. The endocardium lines the interior of the heart chambers, while the myocardium is the thick, muscular layer responsible for the heart’s pumping action. The pericardium is the fibrous sac surrounding the heart, providing additional protection but is not part of the heart's muscle layers.