B. Pericardium
The muscle wall of the heart is surrounded by a thin but double membrane, the pericardium. The innermost or visceral layer of the pericardium is a secretory membrane that is closely attached to the surface of the heart muscle. The outer or parietal layer of the pericardium consists of a thin but tough layer. Figure 1 shows the parietal pericardium (cut). The cells of the pericardium secrete minute quantities of a lubricating fluid that separate the two layers of the pericardium. This pericardial fluid may be increased by diseases creating a pericardial effusion. Cells which together tissues an aggregation of specialized perform certain special functions.

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• I. ANATOMIC FEATURES
  Major structural parts of the heart include the muscular wall (myocardium), the inner lining (endothelium), the outer lining (pericardium), and the blood vessels supply¬ing the heart with blood. The coronary arteries and veins run along the surface of the heart through the pericardium and traverse the muscular wall. Anatomical features of the heart and circulation are illustrated in Figs. 1–10.
• II. CIRCULATION OF BLOOD A. The Heart is a Simple Pump
  The human heart is a muscular pump. Its function is to pump blood containing oxygen, glucose, protein, fat, and salts to every organ, tissue, and living cell of the body. The heart is divided into four chambers. The upper chambers are called the right and left atrium, and the lower chambers are called the right and left ventricles. Blood from all parts of the body drains into veins that empty into the right atrium. Blood passes from the right atrium through the tricuspid valve and reaches the right ventricle. During contraction of the right ventricle, blood is pushed into the lungs where it gives off CO2, takes up oxygen, and returns via the pulmonary veins to the left atrium (Fig. 4).
• BIBLIOGRAPHY
  Gaudin, A. J., and Jones, K. C. Human Anatomy and Physiology. Harcourt Brace Jovanovich, San Diego, 1989.