164 STUDIES IN ADVANCED PHYSIOLOGY. 



is but the sound caused by the "slamming shut" of the 

 doors which lead from the pulmonary artery and aorta back 

 into the heart. 



It will be noticed that the heart in its beat is at rest dur- 

 ing a greater period than it is at work. The auricles work 

 but one part in ten, the ventricles but three parts in ten, 

 while the entire heart counted as a unit would work but 

 four parts, and be at complete rest, that is, passively re- 

 laxed, during six parts of the time. If the chest wall had 

 been removed and the heart exposed, changes in its form 

 might easily have been observed. During the contraction 

 the base of the heart would have changed somewhat from 

 an oval form to a rounded form, while the apex would 

 have been drawn toward the base and become blunter. A 

 material change in the position of the heart does not occur. 



Such is, in a general way, the series of events in one 

 beat. L/et us now turn to the individual events in detail. 



The Filling of the Heart. 



Each heart beat consists of two periods: A period of 

 contraction, which is called the systole, and a period of re- 

 laxation called the diastole. Thus we have an auricular 

 systole of one-tenth and a diastole of nine-tenths of a beat 

 period, and a ventricular systole of three-tenths and a ven- 

 tricular diastole of seven-tenths. The filling of the heart 

 falls, of course, in the diastole. 



The question naturally arises, as the heart is in a per- 

 fectly relaxed and passive condition, what are the agencies 

 that propel the blood forward and cause it to fill the emp- 

 tied chambers? It was formerly supposed that the pressure 

 of the arterial blood behind the venous blood propelled this 

 backward to the heart. But such a view can easily be shown 

 to be erroneous, for when a vein near to the heart is opened 

 there is no pressure of blood inside it, but there may be 

 actually a tendency to a vacuum, and air be very easily 

 sucked in. This proves beyond question that venous blood 

 is sucked toward the heart and not pushed. No doubt the 



