e mass of the heart and is composed primarily of striated muscle cells. Each of the cardiac muscle cells contain one central elongated nucleus with some central euchromatin and some peripheral heterochromatin. The two atria have a very thin myocardial layer which increases greatly in thickness as you go from the atria to the right ventricle and into the left ventricle. The outer surface of the myocardium, next to the epicardium, is not composed of smooth muscle but is very smooth in texture. The inner surface of the myocardium is rough and is raised into trabeculations. The ventricular papillary muscles, which are for the attachment of the chordae tendinae, are extensions of the myocardium even though they are covered by endocardium. The outer layer of the myocardium is superficial bulbospiral and swirls around the ventricle in a clockwise fashion. The middle layer is circular muscles that are the ventricular constrictors. The inner layer, which is deep bulbospiral, swirls around the ventricle in a counterclockwise fashion. The layer underneath the myocardium is known as the enodcardium. It contains a continuous smooth endothelial layer that covers all the inner surfaces of the heart, including the valves. The outer layer of the endocardium, underneath the myocardium, is irregularly arranged collagenous fibers that may contain Purkinje fibers/cells. The inner part of the endocardium contains more regularly arranged collagen and elastic fibers than the outer layer. Some myofibroblasts are present in the endocardium which is thicker in the atria than in the ventricles. There is a subendothelial component of the endocardium underneath the endothelium. The component contains fibroblasts, scattered smooth muscle cells, elastic fibers, collagen fibers, and an amorphous ground substance that contains glycoproteins and proteoglycans. The valves of the heart are attached to the cardiac skeleton and consist of chondro...
