Which of the following is the layer of the heart that is thickest?

The myocardium is the thickest layer of the heart wall and is primarily composed of cardiac muscle tissue. This muscular layer is responsible for the heart's pumping action, facilitating the contraction and relaxation necessary to circulate blood throughout the body. The thickness of the myocardium is particularly prominent in the ventricles, especially in the left ventricle, which needs to generate sufficient pressure to pump blood to the entire body.

In contrast, the endocardium is the innermost layer of the heart and consists of a smooth lining that helps to reduce friction as blood flows through the heart chambers. The epicardium, or visceral layer of the serous pericardium, serves as a protective outer layer and is relatively thin compared to the myocardium. The pericardium is a fibrous sac that surrounds the heart and contains a small amount of fluid to reduce friction but does not contribute to the heart's muscular structure. Thus, the myocardium's unique composition and thickness are essential for its role in cardiac function.