The heart during systole, becoming smaller, generally withdraws from the chest wall except for the apex for the reasons explained above. The effect of this withdrawal on the chest wall can be observed as an inward movement of the chest wall during systole called "retraction." Although the heart is basically comprised of two separate pumps (right and left ventricles), these two pumps operate normally at two vastly different pressures. Left ventricular systolic pressures being approximately five times higher than that of the right ventricle, its wall tension is much higher, resulting in the increased wall thickness of the left ventricular chamber. The effect of the increased muscle mass on the left side leads to dominance of the left-sided hydrodynamic forces described above. This results in the left ventricular apex as the only area of normal contact during systole. The rest of the heart essentially retracts from the chest wall. In a normal heart, this retraction of the chest wall can be observed to be located medial to the apical impulse and involving part of the left anterior chest wall (20). Even the right ventricle, which is anatomically an anterior structure, is normally pulled away from the chest wall because of its own contraction (becoming smaller) and, more importantly, the septal contraction also pulling the right ventricle posteriorly. This retraction observed in normal patients is located medial to the apical impulse (23,24). It can be best appreciated with patients in the left lateral decu-bitus position with a palpating finger only on the apical impulse with clear view of the rest of the precordium for proper observation of the inward movement of the retraction (opposite in direction to the outward movement of the apical impulse). This "medial retraction" identifies and indicates that the left ventricle forms the apical impulse. The extent of the area of medial retraction may be variable depending on both cardiac and extracardiac factors such as the compliance of the chest wall. It may sometimes be noted only over a small area very close to the apex beat.