d the aortic opening. They are similar in structure and mode of attachment to those of the pulmonary artery. However, they are larger, thicker, and stronger than those of the right side. Between each valve and the cylinder of the aorta is a deep depression called the sinuses of Valsalva. The depressions are larger than those at the root of the pulmonary artery.3Figure 1: a. Cross sectional view of the heart. b. Top view of the heart showing the four valvesHistology of the Layers of the Heart: The heart and its vessels are surrounded by a conical membranous sac called the pericardium. The pericardial sac is composed of two layers; the parietal pericardium and the visceral pericardium with the space in-between the two being called the pericardial cavity. The parietal pericardium is composed primarily of compact fibrocollagenous tissue along with elastic tissue. It is a fibrous membrane of loose irregular connective tissue that is lined internally by a mesothelium which is essentially simple squamous epithelium. The visceral pericardium forms the internal lining of the pericardium and reflects over the outer surface of the heart. This reflection forms the outer layer of the epicardium. The visceral pericardium is also composed of compact fibrocollagenous tissue with elastic tissue but, is smooth mesothelium. The pericardial cavity is located between the parietal and visceral pericardium and contains small amounts of serous fluid. The heart tissue itself can be subdivided into three layers; (from the outside in) epicardium, myocardium, and endocardium. The epicardium is the outermost layer of the heart and consists of a loose connective tissue of fibroblasts, collagen fibers, and adipose tissue. It contains a stroma which houses coronary arteries and veins that are surrounded by a layer of fat. These coronary branches penetrate the myocardium. The myocardium contains the main muscl...
