MOVEMENTS OF THE HEART. 263 



The blood cannot pass back into the veins from the auricles when 

 they contract, because the auricular contraction commences in the 

 bundles of muscle which surround the orifices of the great venous 

 trunks ; and it cannot flow back to the auricles, because, as has been 

 seen, the force of the blood current on its entry into the ventricles, 

 by making tense the cords, doses the valves ; while a backward 

 flow from the large arteries is at once prevented by the current dis- 

 tending 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 auricu- 

 lar beat. More careful examination shows that this single action 

 is composed of different phases of activity and repose, which to- 

 gether 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 con- 

 tract 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 dias- 

 tole, which is commonly spoken of as the passive interval 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 stages : 

 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 register- 

 ing graphically the motions of the auricles and ventricles directly 



