Left atrium and ventricle.   A. Windows have been cut in the anterolateral walls of the left atrium and the left ventricle, leaving the mitral apparatus intact. The smooth and thick endocardium of the atrium is shown. The intimate contact of the anterior atrial wall with the pulmonary trunk and the root of the right and left pulmonary arteries is shown. The mitral chordal apparatus and its attachments to the anterolateral and posteromedial papillary muscles is easily seen. B. Parasagittal section of the heart slightly to the left of the posterior aortic cusp. The right side of the right half of the specimen is shown in the left image; the opposite face, in the right image. In both cuts the left atrium appears as a smooth-walled chamber with a thicker, white endocardium. Tiny openings of Thebesian veins are seen, particularly in the posterior wall. The left ventricle is divided anatomically into inflow, apical and outflow portions. The angle between the inflow and the outflow tracts is more acute than that found in the right ventricle. The inflow tract is funnel-shaped and surrounded by the mitral valve annulus, leaflets and chordae tendineae. The apical portion, including the apical two-thirds of the interventricular septum, has fine trabeculations. The apex is normally the thinnest portion of the left ventricle. The outflow tract is formed by the smooth basal portion of the muscular septum, the anterior ventricular wall and the anterior mitral leaflet. The muscular septum bulges into the left ventricular outflow tract. This sigmoid shape of the muscular septum becomes prominent with increasing age. The interventricular septum also has a membranous portion below the right and posterior (noncoronary) aortic cusps (arrowheads­). This area is variable in size. Generally it is divided into an atrioventricular portion, between the right atrium and left ventricle, and an interventricular portion, by the insertion of the septal leaflet of the tricuspid valve. In contrast to the right ventricle, where the tricuspid and pulmonic valves are separated by a muscular band, the left ventricle is characterized by fibrous continuity between the mitral and the aortic valves. The anterior mitral leaflet, being continuous with the left and posterior (noncoronary) aortic cusps, divides the left ventricle into inflow and outflow portions. The mitral valve leaflets are anchored by the anterolateral and posteromedial papillary muscles. The aortic root clearly shows the sinotubular junction, a ridge formed at the border between the sinuses and the tubular segments of the ascending aorta.