228 THE HUMAN BODY. 



altered. At the same time the length of the ventricles is 

 lessened, the apex of the heart approaching the base and be- 

 coming 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/ 7 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, how- 

 ever, 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 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 

 attached 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. Hence 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 

 of the aorta and the pulmonary artery are closed, so that no 

 blood can flow back 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 venae cavae; 

 this passes on through the open mitral and tricuspid valves 

 and fills up the dilating ventricles, as well as the auricles. As 

 the ventricles fill, back currents are set up along their walls 

 and these carry up the flaps of the valves so that by the end 

 of the pause they are nearly closed. At this moment the au- 

 ricles contract, and since this contraction commences at and 



