MOVEMENTS OF THE HEART. 185 



through the patent auriculo-ventricular orifices into the ven- 

 tricles. When the auricles have become fully distended, they 

 contract quickly and with considerable power (the auricular 

 systole), and force the blood into the ventricles, effecting the 

 complete diastole of these cavities. During this contraction, 

 the blood not only ceases to flow in from the veins, but some 

 of it is regurgitated, as the orifices by which the vessels open 

 into the auricles are not provided with valves. The size of 

 the auriculo-ventricular orifices is one reason why the greater 

 portion of the blood is made to pass into the ventricles ; and 

 furthermore, during the auricular systole, the muscular fibres 

 which are arranged around the orifices of the veins constrict 

 them to a certain extent, which tends to diminish the reflux of 

 blood. There can be no doubt that some regurgitation takes 

 place from the auricles into the veins, but this prevents the 

 possibility of over-distention of the ventricles. 



It has been shown by experiments that the systole of the 

 auricles is not immediately necessary to the performance of 

 the circulation. M. Marey, 1 in a recent work on the circu- 

 lation, cites an experiment of Chauveau in which the con- 

 tractility of the auricles was temporarily exhausted by pro- 

 longed irritation ; nevertheless the ventricles continued to act 

 and keep up the circulation. 



Action of the Ventricles. Immediately following the 

 contraction of the auricles, which has the effect of producing 

 complete distention of the ventricles, we have the contraction 

 of the ventricles. This is the chief active operation performed 

 by the heart, and is generally spoken of as the systole. As 

 we should expect from the great thickness of the muscular 

 walls, the contraction of the ventricles is very much more 

 powerful than that of the auricles. By their action, the blood 

 is forced from the right side to the lungs by the pulmonary 

 artery, and from the left side to the system by the aorta. 

 Kegurgitation into the auricles is effectually prevented by the 



1 MAREY, Circulation du Sang, Paris, 1863, p. 36. 



