THE CIRCULATION OF THE BLOOD 183 



semilunar valves are instantly closed to prevent a 

 return of blood to the ventricles from the aorta and 

 pulmonary artery; the period of rest occurs, following 

 the emptying of the ventricles, and the cycle is repeated. 

 The Cardiac Cycle or Revolution. This has been 

 shown above to consist of (1) an auricular contraction, 

 (2) ventricular contraction, (3) the period of repose, 

 during which time the auricles and ventricles are at 

 rest. There are 72 cycles per minute made by the 

 heart of a healthy adult, and the average duration of 

 each cycle is about eight-tenths of a second, divided 

 as follows: 



Auricular systole T V Auricular diastole 7% 



Ventricular systole -*$ Ventricular diastole T 5 <y 



Common pause f l d 



The Heart-beats. In a healthy adult the pulsations 

 or cardiac cycles are 72 per minute; in the fetus, 140 per 

 minute; during the first year of life it decreases to 

 128 per minute; during the third year to 95 per minute; 

 from the eighth to the fourteenth year, 84 per minute. 

 It is more rapid in the female, averaging 8 to 10 

 more beats per minute. The pulse of a person lying 

 down is a few beats less than when sitting or stand- 

 ing. Exercise and digestion temporarily increase the 

 number of beats. 



The Heart Sounds. On placing the ear or the 

 stethoscope to the chest wall overlying the heart, or 

 in the fifth intercostal space, 3J inches from the 

 middle of the sternum, two sounds resembling the 

 pronouncing of the syllables lubb-dupp, lubb-dupp, 

 will be heard. They accompany each pulsation of 

 the heart and are called the first and second sounds; 

 the former is dull and long, and occurs with the 

 cardiac systole or contraction; the latter is short and 

 clear and occurs at the commencement of diastole or 

 relaxation of the heart muscle. The cause of the 

 first sound is supposed to be due to the contraction of 



