Figure 1218

Fibrous skeleton of the heart as seen with the two atria removed.

This fibrous network (indicated in blue) serves for the attachment of cardiac muscle; it also serves for the attachment of the cuspid valves between the atria and ventricles and for the semilunar valves of the aorta and the pulmonary artery. The atrioventricular bundle passes from the right atrium to the ventricular septum via the membranous septum of the fibrous skeleton.

opening for atrioventricular bundle (of His)

conus ligament membranous part of interventricular septum fibrous ring of tricuspid valve fibrous ring of pulmonary trunk opening for atrioventricular bundle (of His)

fibrous ring of aorta left fibrous trigone fibrous ring of mitral valve right fibrous trigone branched atrioventricular (A-V) bundle of the cardiac conduction system. The fibrous skeleton provides independent attachments for the atrial and ventricular myocardium. It also acts as an electrical insulator by preventing the free flow of electrical impulses between atria and ventricles.

• An impulse-conducting system for initiation and propagation of electrical impulses for cardiac muscle contraction. It is formed by highly specialized cardiac muscle cells, which generate and conduct electrical impulses rapidly through the heart.

The wall of the heart is composed of three layers: epicardium, myocardium, and endocardium

The structural organization of the wall of the heart is continuous within the atria and ventricles (see Fig. 12.19). The wall of the heart is composed of three layers. From the outside to the inside they are

• Epicardium, consisting of a layer of mesothelial cells on the outer surface of the heart and its underlying connective tissue. The blood vessels and nerves that supply the heart lie in the epicardium and are surrounded by adipose tissue that cushions the heart in the pericardial cavity.

• Myocardium, consisting of cardiac muscle, the principal component of the heart. The myocardium of the ventricles is substantially thicker than that of the atria because of the large amount of cardiac muscle in the walls of the two pumping chambers.

• Endocardium, consisting 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, also called the subendocardial layer, which is continuous with the connective tissue of the myocardium. The impulse-conducting system of the heart (see below) 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 except in the membranous portion. Endocardium lines each surface of the interventricular septum. The interatrial septum

atrial wall myocardium valve ventricular wall coronary sinus circumflex branch of left coronary artery fibrous skeleton

■ xv epicardium

/mitral / valve

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