Heart br The heart is a hollow muscular organ that is somewhat pyramid br shaped and lies within the pericardium in the mediastinum br (Figs. 3.33 and 3.34). It is connected at its base to br the great blood vessels but otherwise lies free within the br pericardium. br Surfaces of the Heart br The heart has three surfaces: sternocostal (anterior), diaphragmatic br (inferior), and a base (posterior). It also has an br apex, which is directed downward, forward, and to the left. br The sternocostal surface is formed mainly by the br right atrium and the right ventricle, which are separated br from each other by the vertical atrioventricular groove br (Fig. 3.34). The right border is formed by the right atrium; br the left border, by the left ventricle and part of the left auricle. br The right ventricle is separated from the left ventricle br by the anterior interventricular groove. br The diaphragmatic surface of the heart is formed br mainly by the right and left ventricles separated by the posterior br interventricular groove. The inferior surface of the br right atrium, into which the inferior vena cava opens, also br forms part of this surface. br The base of the heart, or the posterior surface, is formed br mainly by the left atrium, into which open the four pulmonary br veins (Fig. 3.35). The base of the heart lies opposite the apex. br The apex of the heart, formed by the left ventricle, is br directed downward, forward, and to the left (Fig. 3.34). br It lies at the level of the fifth left intercostal space, 3.5 in. br (9 cm) from the midline. In the region of the apex, the apex br beat can usually be seen and palpated in the living patient.