The heart consists of muscular fibers, and of fibrous rings which serve for their attachment. It is covered by the visceral layer of the serous pericardium (epicardium), and lined by the endocardium. Between these two membranes is the muscular wall or myocardium (Cardiac Muscular Tissue).
The fibers of the heart differ very remarkably from those of other striped muscles. They are smaller by one-third, and their transverse striæ are by no means so well-marked. They show faint longitudinal striation. The fibers are made up of distinct quadrangular cells, joined end to end so as to form a syncytium. Each cell contains a clear oval nucleus, situated near its center. The extremities of the cells have a tendency to branch or divide, the subdivisions uniting with offsets from other cells, and thus producing an anastomosis of the fibers. The connective tissue between the bundles of fibers is much less than in ordinary striped muscle, and no sarcolemma has been proved to exist.