192 



ESSENTIALS OF PHYSIOLOGY. 



pressure exerted by the contracting muscle upon the contained blood 

 rises higher than that in the pulmonary artery. As soon as this 

 happens, the semilunar valves open, and blood flows into the pulmonary 

 artery from the ventricle, which becomes nearly but not quite empty. 

 When the systole of the ventricle ends and its walls relax, the pressure 

 in its cavity very rapidly falls below that in the pulmonary artery ; and 

 the semilunar valves close, effectually preventing any reflux of blood 

 into the ventricle. A fraction of a second later, the .pressure in the 

 ventricle becomes less than that in the right auricle, the auriculo- 

 ventricular valve opens, and blood, which during the ventricular systole 

 has been flowing into the auricle from the veins, again begins to enter 

 the ventricle. The valves open or close with the slightest difference 

 of pressure on either side. During ventricular systole the efficiency 



'Aorta 



FIG. 63. Diagram to show the position of the mitral valves 

 in diastole (1) and in ventricular systole (2). 



A, auricle ; V, ventricle. 



of the tricuspid valve is assisted by the diminution of the cross 

 section of the base of the heart. A similar series of changes takes 

 place simultaneously in the left side of the heart (fig. 63). 



The series of events just described constitutes a cardiac cycle, and 

 occupies on the average a period of 0-8 second, The .cycle may be 

 regarded as beginning with the auricular systole, which lasts O'l 

 second, and is followed by the ventricular systole, which lasts approxi- 

 mately 0-3 second ; during the remainder of the cycle, 0'4 second, the 

 heart is completely relaxed. When the heart is beating infrequently 

 the duration of the cycle is lengthened, and when the heart is beating 

 frequently it is shortened. These differences are due almost entirely 

 to variations in the time occupied by the diastolic pause, the time 

 taken up by the systole of the auricles and ventricles being remarkably 

 constant. 



(3) Cardiac Impulse. If the hand be placed on the chest in man, 



