MOVEMENTS OF THE HEART. 269 



the ventricles, by making tense the cords, closes the valves; 

 while a backward flow from the large arteries is at once prevented 

 by the current distending the semilunar pockets, and thus firmly 

 closing the valves. 



When viewed for the first time, the beat of the heart appears 

 to be a single act, so rapidly does the ventricular follow the 

 auricular beat. More careful examination shows that this single 

 action is composed of different phases of activity and repose, 

 which together make up the cycle of the heart beat. The time 

 occupied by the contraction of the cavities of the heart is called 

 their systole, their period of rest is called diastole. 



The systole of the corresponding cavities of both sides of the 

 heart is exactly synchronous ; that is to say, the two auricles 

 contract simultaneously, and immediately the contraction of the 

 two ventricles follows like that of the two auricles as a single act. 



The auricular and ventricular contractions are separated by 

 so short a space of time that it is not easily appreciable. The 

 rapidly succeeding acts of auricular and ventricular systole are 

 followed by a period during which both auricles and ventricles 

 are in diastole, which is commonly spoken of as the passive in- 

 terval or pause. 



While the auricles are contracting the ventricles are relaxed, 

 and the relaxation of the auricles commences immediately the 

 ventricular contraction begins, so that only for a very short time 

 both auricles and ventricles are contracted. 



The entire cycle or revolution of the heart beat, occupying 

 nearly a second in the healthy adult, may be divided into three 



Auricular systole. 



Ventricular systole. 



General diastole. 



The exact time occupied by each phase of the cycle can only 

 be calculated approximately. This may be done either by regis- 

 tering graphically the motions of the auricles and ventricles 

 directly communicated to levers brought into contact with their 

 surface, or by recording graphically the pressure changes which 

 occur within the cavities by introducing into them little elastic 



