ACTION OF THE HEART 155 



life. Each successive contraction really begins in the great veins, which 

 are muscular, and extends over the auricles and ventricles in regular sequence. 

 The contraction of each successive part is called its systole and the relaxation 

 its diastole. The diastole covers the period of active relaxation of the muscle 

 and the pause before beginning its next contraction. Each muscular cham- 

 ber of the heart may, therefore, be said to have its systole and diastole. The 

 whole series of events from the beginning of one contraction until the cor- 

 responding event in the next contraction is described as the cardiac cycle. 



Action of the Auricles. The description of the action of the heart 

 may be commenced at that period in each cycle in which the whole heart is 

 at rest. The heart is then in a passive state. The auricles are gradually 

 filling with the blood flowing into them from the veins, and a portion of this 

 blood is passing at once through the auricles into the ventricles, the opening 

 between the cavity of each auricle and that of its corresponding ventricle 

 being free during the entire pause. The auricles, however, receiving more 

 blood than at once passes through them to the ventricles, become, near the 

 end of the pause, passively distended. At this moment a contraction wave 

 begins on the bases of the venae cavae and, running down from the walls of 

 the veins, passes to the muscular walls of the auricle. The contraction of the 

 auricles, the right and left contracting at the same time, forces the blood 

 into the ventricles. 



The contraction of the muscular walls of the great veins maintains a 

 condition of constriction of these veins during the time of the auricular con- 

 traction. This hinders the reflux of blood from the auricles into the veins 

 during the auricular systole. Any slight regurgitation from the right auricle 

 is limited by the valves at the junction of the subclavian and internal jugular 

 veins beyond which the blood cannot move backward, and by the coronary 

 vein which is supplied with a valve at its mouth. The force of the blood 

 propelled into the ventricle at each auricular systole is transmitted in all 

 directions, but, being insufficient to open the semilunar valves, it is expended 

 in distending the walls of the ventricle. 



Action of the Ventricles. The dilatation of the ventricles which 

 occurs during the latter part of the diastole of the auricles, is completed by 

 the forcible injection of the contents of the latter. The ventricles, now dis- 

 tended with blood, immediately begin to contract. The tricuspid valves 

 are closed by the initial reflux of blood, or possibly by the currents of blood 

 formed by the sudden injection of the ventricles by the auricular contraction. 

 The ventricular systole follows the auricular systole so closely that it seems 

 continuous with it. As a result of the ventricular systole sufficient pressure 

 is produced on its contents to overcome the pressure against the semilunar 

 valves of the aorta, and the pulmonary artery and the ventricles are thus 

 emptied completely. After the whole of the blood has be",n expelled from 

 the ventricles, the walls remain contracted for a brief period. 



