220 THE HUMAN BODY. 



heart becomes shorter and rounder, mainly from a change 

 in the shape of the ventricles. A cross-section of the heart 

 at the base of these latter during diastole would be ellipti- 

 cal in outline, with its long diameter from right to left: 

 during the systole it is more circular, the long axis of the- 

 ellipse becoming shortened while the dorso- ventral diameter 

 remains little altered. At the same time the length of the 

 ventricles 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 ventri- 

 cles yield to the chest-wall where they touch it, but dur- 

 ing 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 tolerably full, this will be distended not only 

 transversely but longitudinally. 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 at- 

 tached to the heart pushes the latter down towards the dia- 

 phragm, and compensates for the upward movement of the 

 apex due to the shortening of the ventricles. Herice if the 

 exposed living heart be watched it appears as if during the 

 systole the base of the heart moved towards the tip, rather 

 than the reverse. 



Events occurring within the Heart during a Cardiac 

 Period. Let us commence at the end of the ventricular 

 systole. At this moment the semilunar valves at the orifices. 



