The heart communicates its motion to the chest wall, and the movement can be felt and seen over a limited area, which varies with the thinness of the individual. This cardiac impulse, as the stroke is called, can best be felt in the fifth intercostal space, a little to the median side of the left nipple. It is found to be synchronous with the ventricular systole. During this period ventricular systole - the base of the ventricles moves downward and becomes thicker. The flaccid cone which is formed by ventricles during diastole is somewhat flattened against the chest wall, but during systole it becomes rounded and bulges forward, pushing the chest wall before it. This change in shape is the chief cause of the cardiac impulse.
If the ventricles be gently held between the "fingers during their systole, a most striking sensation is given by the change of shape and the sudden hardening of the muscle. The mass in the ventricles, from being quite soft and compressible during diastole, suddenly acquires a wooden hardness, owing to the tightness with which the muscle grasps the fluid, and the greater firmness of the contracting tissue.
This hardening gives the sensation of a sudden enlargement. No matter on what surface the finger be placed, the heart seems to give a slight knock in that direction. Thus, when grasped between the forefinger placed below the diaphragm and the thumb on the antero-superior aspect, the impulse is equally felt by each digit.
The important items in causing the impulse are, then, the change in shape of the ventricles from a flattened to a rounded cone, and their simultaneous hardening, which no doubt helps to make the movement more distinctly felt through the wall of the chest.
The point at which the impulse is best felt corresponds to the anterior surface of the ventricles at a considerable distance above the apex; it is therefore erroneous to call the impulse the "apex beat".
The cardiac impulse is a valuable measure of the strength of the systole, and hence is of great importance to the clinical physician. It may be registered by means of an instrument called the Cardiograph. Many such instruments have been devised, most of which work on the same principle, and make a record on a moving surface with a lever attached to a tambour, to which the movements of the chest wall are transmitted from a somewhat similar drum by means of air tubes. In using this plan, so generally employed by Marey, one air tambour (Fig. 120) is applied over the heart, the motions of which cause a variation in the tension of the air it contains; these variations are transmitted by a tube, f (Fig. 121), to the other tambour (3), where they give rise to a motion in its flexible surface, to which a delicate lever is attached at (a).