Heart. The base of the heart corresponds posteriorly to the fifth, to the ninth thoracic vertebra. Anteriorly the apex is in the fifth intercostal space, 7.5 to 8 cm. (3 to 3.25 in.) from the median line. The base (above) corresponds to a line (A) drawn from a point 1 cm. (0.4 in.) below the second left chondro-costal articulation, and 3 cm (1.2 in. from the median line, to another point (the same distance from the median line) 1 cm. above the right third chondro-sternal articulation. The marge acutus, or lower border corresponds to a line (B) drawn from the apex through the xiphi-sternal articulation, to a point on the sixth costal cartilage 2 cm. to the right of the median line. The right border of the heart may be indicated approximately by a fine (slightly convex to the right) joining the right ends of A and B. The left border corresponds to a fine (slightly convex to the left) joining the left end of A to the apex.

If a line be drawn from the upper margin of the left third chondro-sternal articulation to the right edge of the sternum in the fifth intercostal space, the upper end of the line will he over the center of the pulmonary ostium, and the lower two-thirds of it (approximatively will overlie the main axis of the tricuspid ostium. The aortic ostium is immediately to the left of the above line with its center at the left edge of the sternum opposite the third space. The mitral ostium is very largely behind the third left interspace; its upper end is behind the third cartilage, its lower behind the left margin of the sternum opposite the fourth cartilage and space.

Of the ostia of the heart, the pulmonary is nearest the anterior thoracic wall, the aortic is slightly in advance of the mitral, and the tricuspid is the deepest of all.

The pericardium follows the heart closely. The upper end (apex) in a subject can extend up, behind the sternum, to the lower margin of the first costal cartilage on the right and the upper margin of the second on the left.