What is the thickest layer of the heart?

The myocardium is the thickest layer of the heart, primarily composed of cardiac muscle tissue. This muscular layer is crucial for the heart's function, as it is responsible for the contractions that pump blood throughout the body. The thickness of the myocardium varies in different parts of the heart, being particularly thick in the ventricles, which need to generate strong contractions to circulate blood effectively.

The endocardium, while important, is the thin inner lining of the heart that provides a smooth surface for blood flow. The epicardium, also known as the visceral layer of the pericardium, is a thin layer covering the heart. Lastly, the pericardium is a protective sac surrounding the heart, which contains fluid to reduce friction during heartbeats but does not contribute to the heart's pumping ability like the myocardium. Thus, the myocardium's thickness is essential for its role in cardiac function, making it the correct answer.