FIGURE 13.7 • Horizontal section through the ventricles of the heart. This photograph shows a cross section of the human heart at the level of the ventricles. Cusps of both the tricuspid valve in the right ventricle and the mitral valve in left ventricle are visible with their attachments to the chordae tendineae. Cross sections of the papillary muscles in both ventricles are visible. Note the differences in the thickness between the wall of the right and left ventricles. Adipose tissue of the epicardium contains branches of the coronary arteries and tributaries of the coronary veins. RV, right ventricle; LV, left ventricle. (Courtesy of Dr. William D. Edwards, Mayo Clinic, Rochester, MN.)
FIGURE 13.8 • Photomicrograph of the left atrial and left ventricular walls. a. This photomicrograph shows a sagittal section of the posterior wall of the left atrium and left ventricle. The line of section crosses the coronary (AV) groove containing the coronary sinus and circumflex branch of the left coronary artery. Note that the section has cut through the fibrous AV ring of the mitral valve, which provides the attachment site for the muscle of the left atrium and the left ventricle and the cusp of the mitral valve. The ventricular wall consists of three layers: (1) endocardium (arrowheads), (2) myocardium, and (3) epicardium. The visible blood vessels lie in the epicardium and are surrounded by adipose tissue. The layers of the mitral valve are shown at higher magnification in Figure 13.9b. 35.
b. This high magnification of the area indicated by the rectangle in a shows the characteristic features of the inner surface of the heart. Note that the endocardium consists of a squamous inner layer of endothelium (End), a middle layer of subendothelial dense connective tissue (DCT ) containing smooth muscle cells (SMC), and a deeper subendocardial layer containing Purkinje fibers (PF). The myocardium contains cardiac muscle fibers (CMF) and is seen on the left. 120.
large numbers of proteoglycans. The spongiosa acts as a shock absorber to dampen vibrations associated with the closing of the valve. It also confers flexibility and plasticity to the valve cusps. In the aortic and pulmonary valves, spongiosa located on the blood vessel side is called arterialis. It corresponds to the loose connective tissue located on the atrial side of the AV (tricuspid and mitral) valves, which is called the auricularis.
 The ventricularis is immediately adjacent to the ventricular or atrial surface of each valve and is covered with endothelium. It contains dense connective tissue with many layers of elastic fibers. In the AV valves, the ventricularis continues into the chordae tendineae, which are fibrous, threadlike cords also covered with endothelium (Fig. 13.9). They extend from the free edge of the AV valves to muscular projections from the wall of the ventricles, which are called papillary muscles. Valve cusps are normally avascular. Small blood vessels and smooth muscle can be found only in the base of the cusp. The surfaces of the valve are exposed to blood, and the cusps are thin enough to allow nutrients and oxygen to diffuse from the blood.