Fig. 23-21. Horizontal section showing all four chambers of the heart. The left atrium is almost entirely on the back of the heart. The interventricular and interatrial septa are at approximately a 45-degree angle to the median plane. The "right heart" (blue arrow) lies in front of the "left heart" (red arrow). The outline of the arch of the aorta (with the origin of its three branches), as seen from above, has been included, although it would lie about three vertebrae higher than the plane of this section. The position of a film for a left anterior oblique (L.A.O.) projection is shown. In such a view (shown in miniature next to the film), the right and left portions of the heart would appear approximately equally separated along the line of the interventricular septum. Moreover, because the arch of the aorta passes almost directly backward, rather than transversely, its curve would appear well opened out. A valuable article on cardiac anatomy is that by R. Walmsley and H. Watson (Br. Heart J., 28:435-447, 1966). B.T., brachiocephalic trunk; C.C., left common carotid artery; I.V.S., interventricular septum; L.A., left atrium; L.5., left subclavian artery; L.V., left ventricle; M., mitral valve; R.A., right atrium; R.V., right ventricle; T., tricuspid valve; T.V., thoracic vertebra. Click for high resolution image.