THE HEART 43 



Contraction. The physiological contraction of the cardiac 

 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 con- 

 traction passes on. The whole time of contraction, from one 



FIG. 22. Scheme of cardiac cycle. 



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

 tion of the sounds and pauses to these events. (Kirkes after Sharpey and Gairdner.) 



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 (systole) 

 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 auricles and ventricles 

 then rest and this occupies .4 second. 



Number of Beats. In an adult the heart beats on an average 

 of 72 times per minute, in children it is higher. The frequency 

 of beat is influenced by age, sex, disease, drugs, physical causes 

 and digestion. 



