The muscular layer of the heart is called the myocardium. It is the thickest part of the heart and the middle of the heart wall's three layers, the other two being the epicardium, a thin layer of fat, and the endocardium, a non-adherant surface on the interior of the heart.
The myocardium is composed of cardiomyocytes, which are specialized cardiac muscle cells that control the contraction of the heart by generating and transferring electricity. These cardiac muscle cells require a dedicated continuous blood supply to maintain proper function. The coronary arteries serve this purpose by supplying the myocardium with blood.