THE HEART 95 



tion is technically known as a systole, and each relaxation as 

 a diastole. During the systoles of the ventricles, which occur 

 simultaneously, the blood is forced into the arteries, because 

 the cavity of the ventricles is diminished in size, and, as the 

 auriculoventricular valves are closed, the blood must pass 

 through the open semilunar valves. During diastole the semi- 

 lunar valves are closed, preventing the regurgitation of the 

 blood from the arteries, but the auriculoventricular valves 

 are now open, so that the blood in the large veins and in the 

 auricles can enter the ventricles. 



The beat of the heart consists of a regular sequence of events 

 known as the cardiac cycle. The systoles of the two auricles 

 occur together, as do those of the ventricles, and the same 

 is true of their diastoles. While the auricles are contracting 

 they shrink in size, and at this time the ventricles swell. Then 

 follow immediately the systoles of the ventricles, during which 

 the ventricles diminish in size, the auricles swell, and the 

 injected arteries grow larger and longer. During the succeed- 

 ing diastoles of the ventricles both ventricles and auricles 

 swell until the next contraction of the auricles swells the ven- 

 tricles still more. These changes in the size of the heart are 

 due entirely to the varying amounts of blood contained, and 

 not to any variations in the bulk of the heart muscle. In the 

 relaxed condition the heart walls are very soft and flaccid. 

 Owing to this fact the changes of form that the heart under- 

 goes are easily modified by gravity when the thorax is opened 

 and the heart exposed, since it is then unsupported by the 

 lungs, which normally have a dilating influence. In this con- 

 dition in an animal lying on its back it is seen that during a 

 contraction of the ventricle the long axis of the heart sweeps 

 toward the median line, and also toward the head, so that the 

 apex rises a little toward the observer. The heart twists so 

 that the left ventricle moves nearer the breast, while the right 

 turns toward the spine. There is no change of color in the 

 ventricle, but the auricles being thin, show the blood within 

 the right, bluish; the left, bright red. As the auricles contract 

 they become paler. A distinct pulse is present in the arteries. 

 In the unopened chest it is probable that the ventricles become 

 smaller in girth during systole, and that they are always approxi- 

 mately circular and not elliptical. The ventricles shorten 



