The heart wall consists of three layers: the endocardium, myocardium and epicardium.
The endocardium is the thin membrane that lines the interior of the heart.
The myocardium is the middle layer of the heart. It is the heart muscle and is the thickest layer of the heart.
The epicardium is a thin layer on the surface of the heart in which the coronary arteries lie.
The pericardium is a thin sac the heart sits in, often filled with a small amount of fluid, which separates the heart from the other structures in the chest such as the lungs.