Gross Anatomy of the atria.   A. Caudal view of the atria. Both atria have been dissected at the level of their insertion into the fibrous annulus of the heart. The right atrium has a thin and translucent endocardium, whereas the left atrial endocardium is white and thick. The pectinate muscles are prominent throughout the posterior and lateral walls of the right atrium. They merge into the crista terminalis. The Eustachian valve is prominent and borders the opening of the inferior vena cava into the right atrium. B. Another caudal view of the atria, also showing the great arteries. The great vessels are transected just above the plane of the semilunar valves. This view shows the intimate relationship of the aorta to the anterior right atrial wall and the interatrial septum. The ascending aorta is located dorsal and to the right of the pulmonary trunk. C. Dorsocephalic view of the heart. The pulmonary veins are seen entering the left atrium dorsally. The openings of the superior and inferior venae cavae are seen flanking the sulcus terminalis (arrows). D. View, (similar to that in C) of the heart of an elderly patient. Dilatation of the atria and the great vessels is prominent. Atria are viewed from the posterior or basal portion of the heart in the anatomic position. The left atrium is more posterior than the right atrium. The right pulmonary veins enter the left atrium by passing behind the right atrium. The superior and inferior venae cavae enter into the posterior segment of the right atrium. The pulmonary trunk and its bifurcation overlie the ventral surface of the left atrium. The ligamentum arteriosus (LiA) normally arises from the left pulmonary artery close to the bifurcation and inserts into the aorta just distal to the origin of the left subclavian artery