 The myocardium, consisting of cardiac muscle, is the principal component of the heart. The detailed histologic structure and function of cardiac muscle is discussed in Chapter 11, Muscle Tissue. The myocardium of the atria is substantially thinner than that of the ventricles. The atria receive blood from the large veins and deliver it to adjacent ventricles, a process that requires relatively low pressure. The myocardium of the ventricles is substantially thicker because of the higher pressure required to pump the blood through the pulmonary and systemic circulations (Fig. 13.7).
 The endocardium consists of an inner layer of endothelium and subendothelial connective tissue, a middle layer of connective tissue and smooth muscle cells, and a deeper layer of connective tissue, which is also called the subendocardial layer. The latter is continuous with the connective tissue of the myocardium. The conducting system of the heart (see the following section called “Intrinsic Regulation of Heart Rate”) is located in the subendocardial layer of the endocardium.
The interventricular septum is the wall between the right and left ventricles. It contains cardiac muscle in all but the membranous portion. Endocardium lines each surface of the interventricular septum. The interatrial septum is much thinner than the interventricular septum. Except in certain localized areas that contain fibrous tissue, it has a center layer of cardiac muscle and a lining of endocardium facing each chamber.
FIGURE 13.6 • Layers of the heart and pericardium. This schematic diagram shows the anatomical relationship between the layers of the heart. In the middle mediastinum, the heart and roots of the great vessels are surrounded by the pericardium, which is often covered by highly variable amounts of adipose tissue. The pericardium has two layers: a tough external fibrous layer called the fibrous pericardium and a parietal layer of serous pericardium that lines its inner surface. The parietal layer of the serous pericardium is reflected back at the great vessels entering and leaving the heart as the visceral layer of the serous pericardium or epicardium. The epicardium lines the outer surface of the heart. The pericardial cavity is a space between the visceral and parietal layers of the serous pericardium, and it is lined by the mesothelial cells. Deep to the epicardium is the myocardium consisting of cardiac muscle. Note a small amount of adipose tissue of the epicardium, which contains the coronary arteries and cardiac veins. The inner layer of the myocardium is called the endocardium, which is lined by the mesothelium with an underlying thin layer of connective tissue.
Heart valves are composed of connective tissue with overlying endocardium.
The heart valves attach to the complex framework of dense irregular connective tissue that forms the fibrous rings and surrounds the orifices containing the valves (Fig. 13.8). Each valve is composed of three layers.
 The fibrosa forms the core of the valve and contains fibrous extensions from the dense irregular connective tissue of the skeletal rings of the heart.
 The spongiosa is loose connective tissue located on the atrial or blood vessel side of each valve. It is composed of loosely arranged collagen and elastic fibers infiltrated with