MECHANICS OF THE HEART-BEAT 87 



merits becomes closer and more extensive; and their tendency to 

 belly into the auricles is opposed by the pull of the chordae tendineae, 

 whose slender cords, inserted into the valves from border to base, are 

 kept taut, in spite of the shortening of the ventricles by the con- 

 traction of the papillary muscles. The arrangement and connec- 

 tions of the muscular fibres of the heart are such that during the 

 auricular systole the auriculo-ventricular groove moves towards the 

 base of the heart, while during the systole of the ventricles it moves 

 towards the apex, which constitutes a relatively fixed point on 

 account of the mutual action of the numerous fibres which converge 

 here and constitute the ' whorl.' The line joining the apex and 

 the origin of the aorta does not shorten when the ventricles contract, 

 but all parts of the heart are drawn towards this line. The apex is, 

 therefore, pushed forwards, while the rest of the ventricular surface 

 is being drawn inwards. During the systole, the ventricles change 

 their shape in such a way that their combined cross-section which 

 in the relaxed state is a rough ellipse with the major axis from right 

 to left becomes approximately circular, and they then form a right 

 circular cone. As soon as the pressure of the blood within the con- 

 tracting ventricles exceeds that in the aorta and pulmonary artery 

 respectively, the semilunar valves, which at the beginning of the 

 ventricular systole are closed, yield to the pressure, and blood is 

 driven from the ventricles into these arteries. 



The ventricles are more or less completely emptied during the 

 contraction, which seems still to be maintained for a short time after 

 the blood has ceased to pass out. The contraction is followed 

 by sudden relaxation. The intraventricular pressure falls. The 

 lunules of the semilunar valves slap together under the weight of the 

 blood as it attempts to regurgitate, the corpora Arantii seal up the 

 central chink, and the aorta and pulmonary artery are thus cut off 

 from the heart. Then follows an jnterval during which the whole 

 heart is at rest, namely, the interval between the end of the relaxa- 

 tion of the ventricles and the beginning of the systole of the auricles. 

 This constitutes the pause. The whole series of events is called a 

 cardiac cycle or revolution (see Practical Exercises, p. 201). 



It will be easily understood that the time occupied by any one of 

 the events of the cardiac cycle is not constant, for the rate of the 

 heart is variable. If we take about 70 beats a minute as the average 

 normal rate in a man, the ventricular systole will occupy about 

 0-3 second; the diastole,* including the ventricular relaxation, about 



* The term ' diastole ' is variously used, as meaning the pause, the pause 

 plus the period during which relaxatidn is occurring, or the period of re- 

 laxation alone. We shall employ it in the second sense. Henderson refers 

 to the period during which the ventricular muscle is at rest, from the end of 

 its relaxation to the onset of the auricular systole, as the ' diastasis ' and the 

 period during which the relaxation is taking place as the 'diastole,' a termin- 

 ology which seems worthy of general adoption. 



