my·o·car·di·um / ˌmīəˈkärdēəm/ • n. Anat. the muscular tissue of the heart.DERIVATIVES: my·o·car·di·al / -dēəl/ adj.

myocardium (my-oh-kar-diŭm) n. the middle of the three layers forming the wall of the heart (see also endocardium, epicardium). It is composed of cardiac muscle and forms the greater part of the heart wall.
—myocardial adj.

myocardium The muscular wall of the heart. The thickness of the myocardium varies, reflecting the magnitude of the pressure generated in the heart during contraction: the myocardium is thickest around the left ventricle, which does most of the work in the heart. The inner surface of the myocardium is lined by a layer of endothelium (the endocardium), which is continuous with the endothelium lining the blood vessels.