76 A MANUAL OF PHYSIOLOGY 



closer and more extensive ; and their tendency to belly inta 

 the auricle is opposed by the pull of the chordae tendinese, 

 whose slender cords, inserted into the valves from border to 

 base, are kept taut, in spite of the shortening of the ventricle 

 by the contraction of the papillary muscles. 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 begin- 

 ning 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 intra- 

 ventricular 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 interval during 

 which the whole heart is at rest, namely, the interval 

 between the end of the relaxation 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. 176). 



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 '3 second ; the ventricular diastole, 

 including the relaxation, about '5 second. The systole of 

 the auricle is one-third as long as that of the ventricle. 



This rhythmical beat of the heart is the ground phe- 

 nomenon of the circulation. It reveals itself by certain 

 tokens sounds, surface-movements or pulsations, alterations 

 of the pressure and velocity of the blood, changes of volume 

 in parts all periodic phenomena, continually recurring 



