What are the muscle walls of the heart called?

The muscle walls of the heart are called the myocardium. The other options are incorrect because A: The pericardium is the outer layer of tissue that surrounds the heart, not the muscle walls. C: The endocardium is the inner layer of tissue that lines the chambers of the heart, not the muscle walls. D: The epicardium is the layer of tissue that covers the surface of the heart, not the muscle walls.