340 THE HUMAN BODY 



Change of Form of the Heart. During its systole the heart 

 becomes shorter and rounder, mainly from a change in the shape 

 of the ventricles, which from having an elliptical cross-section 

 take on a circular one. At the same time the length of the ven- 

 tricles is lessened, the apex of the heart approaching the base and 

 becoming blunter and rounder. 



The Cardiac Impulse. The human heart lies with its apex 

 touching the chest-wall between the fifth and sixth ribs on the 

 left side of the breast-bone. At every beat a sort of tap, known 

 as the "cardiac impulse" or "apex beat," may be felt by the 

 finger at that point. There is, however, no actual "tapping, 

 since the heart's apex never leaves the chest-wall. During the 

 diastole the soft ventricles yield to the chest-wall where they 

 touch it, but during the systole they become hard and tense and 

 push it out a little between the ribs, and so cause the apex beat. 

 Since the heart becomes shorter during the ventricular systole, it 

 might be supposed that at that time the apex would move up a 

 little in the chest. This, however, is not the case, the ascent of 

 the apex towards the base of the ventricles being compensated 

 for by a movement of the whole heart in the opposite direction. 

 If water be pumped into an elastic tube, already moderately full, 

 the tube will be distended not only transversely but longitudi- 

 nally. This is what happens in the aorta: when the left ventricle 

 contracts and pumps blood forcibly into it, the elastic artery is 

 elongated as well as widened, and the lengthening of that limb of 

 its arch attached to the heart pushes the latter down towards the 

 diaphragm, and compensates for the upward movement of the 

 apex due to the shortening of the ventricles. Hence if the ex- 

 posed living heart be watched it appears as if during the systole 

 the base of the heart moved towards the tip, rather than the re- 

 verse. 



Events occurring within the Heart during a Cardiac Cycle. 

 Let us commence at the end of the ventricular systole. At this 

 moment the semilunar valves at the orifices of the aorta and the 

 pulmonary artery are closed, so that no blood can flow bajck from 

 those vessels. The whole heart, however, is soft and distensible 

 and yields readily to blood flowing into it from the pulmonary 

 veins and the venee cavse; this passes on through the open mitral 

 and tricuspid valves and fills up the dilating ventricles, as well as 



