If the diagram representing the frontal view of the heart outlines, as shown on the fluoroscopic screen or by roentgenograms, is studied, several curves and depressions are noticed. The right side of the heart presents a single convex curve caused almost wholly by the auricle; the left side presents usually two curves, the superior curve caused by the aorta and the greater and inferior curve caused by the left ventricle. Between these two curves, situated usually between the second and third ribs, is a depressed line or concavity.
In this area, in abnormal conditions of the heart, the pulmonic artery and a portion of the auricle below it can sometimes cause two smaller curves instead of the normal depression. Or, frequently, a single marked curve is noticeable, so that the left side of the heart may show three noticeable curves instead of two. This third curve, if it covers the
HONEIJ JA. THE DILATATION OF THE PULMONIC AREA OF THE HEART AND ITS SIGNIFICANCE. Arch Intern Med (Chic). 1923;31(6):866-870. doi:10.1001/archinte.1923.00110180087008