THE CIRCULATION OF THE BLOOD. 



275 



These three phases collectively constitute a cardiac cycle or a 

 cardiac revolution. The duration of a cycle, as well as the duration 

 of each of its three phases, varies in different animals in accordance 

 with the number of cycles which recur in a unit of time. In human 

 beings in adult life there are about 72 cycles to the minute; the average 

 duration therefore is 0.83 second. From this it follows that the time 

 occupied by any one of the three phases must be extremely short and 

 difficult of determination. From observations made on human 

 beings and from experiments on animals the following estimates 

 have been made and accepted as approximately correct: 



1. The auricular systole, 0.16 second. 



2. The ventricular systole, 0.32 second. 



3. The period of rest for both auricles and ventricles, 0.32 second. 

 The relations of these three phases to one another may be illustrated 



by the following diagram (Fig. 121), in which the space 1-2 is the 

 duration of a cardiac cycle di- 

 vided into eight equal spaces, 

 each of which represents one- 

 tenth of a second. The line A 

 represents the auricular, the line 

 V the ventricular phase. The 

 rise in the line A represents the 

 contraction; the fall and subse- 

 quent continuation, the relaxa- 

 tion and pause. The rise in the 

 line V and its continuation rep- 

 resent the contraction; the fall 

 and subsequent continuation, the 

 relaxation and the pause. From 

 this it is apparent that the auric- 

 ular contraction or systole has a 

 brief duration, 0.16 second, while the relaxation or diastole has a 

 long duration, 0.64 second; that the ventricular contraction immedi- 

 ately following the auricular has a duration of 0.32 second, while the 

 relaxation and diastole have a duration of 0.48 second; that the pause 

 of the entire heart, that is, the period between the termination of the 

 ventricular systole and the beginning of the next auricular systole, is 

 only 0.32 second. 



The frequency of the heart-beat varies with a variety of con- 

 ditions: e. g., age, sex, posture, exercise, etc. 



Age. The most important normal condition which modifies the 

 activity of the heart is age. Thus : 



Before birth, the number of beats a minute averages 14 



During the first year it diminishes to 128 



During the third year it diminishes to 95 



From the eighth to the fourteenth year it averages 84 



In adult life it averages 7 2 



FIG. 121. THE PHASES or THE HEART'S 

 PULSATION. 



