THE HEART 43 



Contraction. The physiological contraction of the car- 

 diac muscle is called systole, the relaxation is called diastole. 

 The contraction of the heart starts at the mouth of the 

 veins and, with a uniform rhythm glides along through the 

 auricles and along to the ventricles, each part relaxing as the 

 rhythmic contraction passes on. The whole time of contrac- 



OIASTOLE 

 OF 



UR1CLE&VENTRICLE. 



FIG. 22. Scheme of cardiac cycle. 



The inner circle shows the events which occur within the heart; the outer the 

 relation of the sounds and pauses to these events. (Kirkes after Sharpey and 

 Gairdner.) 



tion, from 'one beginning in the veins to another beginning, 

 is called the cardiac cycle. It lasts about .86 second. 



The cycle may be divided thus : the auricles contract (sys- 

 tole) and ventricles are relaxed (diastole) which occupies 

 .16 second; the ventricles contract (systole) and the auricles 

 are relaxed (diastole) and this occupies .3 second; both au- 

 ricles and ventricles then rest and this occupies .4 second. 



Number of Beats. In an adult the heart beats on an aver- 

 age of 72 times per minute, in children it is higher. The fre- 

 quency of 'beat is influenced by age, sex, disease, drugs, phy- 

 sical causes and digestion. 



