The cardiac borders vary even in healthy individuals, depending on age, posture, pregnancy, etc. On average, the cardiac borders (outline) can be projected on the anterior surface of the trunk in the following manner: the right border extends from the sternal junction of the 3rd rib to the junction of the 6th rib parallel to the right sternal margin and about 2cm from it; the left border lies approximately along a straight line extending from a point 2 cm to the left of the sternal junction of the 3rd rib to a point in the 5th left intercostal space, 2 cm inside the mid-clavicular line. The apex of the heart in the adult lies at this point, but that of a child lies one interspace higher. The interpleural space is interpolated on both sides in front of the pericardium. The lungs extend into it more during the inspiration than expiration, covering the lateral parts of the heart. From the sternal junction of the 4th rib downward the left pleural margin leaves a lower triangular portion of the pericardium uncovered. Above the attachment of the 3rd rib, the pleura separate toward the apices of the lungs and leave a superior triangular retro-sternal zone uncovered, the thymic triangle in which lies the thymus and the thymus remnants which cover the large vessels.