Which part of the heart wall is responsible for the propulsion of blood?

The myocardium is the middle layer of the heart wall and is primarily responsible for the heart's ability to pump blood throughout the body. Composed mainly of cardiac muscle tissue, the myocardium's contraction generates the force needed to propel blood out of the heart chambers. This muscle layer is unique because it exhibits striations similar to skeletal muscle but operates involuntarily, controlled by the autonomic nervous system.

In contrast, the epicardium serves as the outer layer of the heart wall and is comprised of connective tissue and fat; it plays a protective role rather than directly contributing to the heart's pumping action. The endocardium lines the interior of the heart chambers and is involved in providing a smooth surface for blood flow, without participating in muscle contraction. Lastly, the pericardium is a sac-like structure surrounding the heart, providing mechanical support and protection, but it does not contribute to the propulsion of blood.

Therefore, the myocardium is essential for the heart's function and the effective circulation of blood, making it the correct answer.