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Structure of the heart wall, endocardium, myocardium, epicardium. The heart wall itself is made up of the endocardium, myocardium and epicardium. The endocardium is an endothelial membrane that lines the inner surface of the myocardium and extends through the intima of the large vessels. The myocardium, or heart muscle, is the thick, hollow striated muscle that is the part of the heart that is capable of contracting. It is an autonomic muscle regulated by the sympathetic and parasympathetic systems. The ventricular wall is thicker than the atrial wall because the myocardium is larger there. The epicarp is the visceral layer of the serous pericardium. This leaflet envelops the heart. (Photo by: JACOPIN/BSIP/Universal Images Group via Getty Images)