THE MOVEMENTS OF THE HEART. 97 



1. The pressure of the blood in the venae cavae (on the right) and 

 the pulmonary veins (on the left), which is greater than the pressure 

 within the auricles. 



2. The elastic traction of the lungs, which after the completed con- 

 traction tends to separate the relaxed yielding walls of the auricles lying 

 in contact with each other. The auricular appendages are distended 

 coincidently with the auricles. The appendages serve in a measure 

 as reservoirs for the auricles, to accommodate the large amount of 

 blood flowing in from the veins. 



(B) The Auricles Contract. There occur in rapid sequence: 



1. The contraction and evacuation of the auricular appendages in the 

 direction of the auricle. Simultaneously, the entering veins are con- 

 stricted by the contraction of their circular muscular layers, especially 

 the superior vena cava and the site of entrance for the pulmonary 

 veins. 



2. The walls of the auricles contract rapidly in a wave-like manner 

 from above downward, particularly toward the auriculo-ventricular 

 orifices, in consequence of which 



3. The blood is forced downward into the relaxed ventricles, which 

 now become considerably dilated. As a result of the auricular con- 

 traction there occur: 



(a) A slight stasis of the blood in the large venous trunks, such as 

 can be readily observed particularly in rabbits on exposure of the point 

 of junction of the jugular and subclavian veins after division of the 

 muscles of the chest. There is no actual reflux of the blood, but only 

 a slight stasis due to partial interruption of the flow into the auricle, 

 because, as has been stated, the sites of entrance for the veins are nar- 

 rowed; because, further, the pressure in the superior vena cava and in 

 the pulmonary veins soon counteracts the tendency to regurgitation ; 

 and, finally, because in the further ramifications of the inferior and 

 to some extent also of the superior cava and of the cardiac veins, 

 valves prevent the reflux. In the blood thus stagnated in the venae 

 cavae the movement of the heart causes a regular pulsating phenome- 

 non that, when abnormally increased, may give rise to the appearance 

 of a venous pulse. 



(6) The principal motor effect of the auricular contraction is the 

 distention of the relaxed ventricles, which in small measure are dilated 

 by the elastic traction of the lungs. 



Earlier and later investigators have attributed the distention of the ven- 

 tricles in part to the elasticity of the muscular walls. It has been thought that 

 the strongly contracted ventricular walls, like a compressed rubber bulb, in re- 

 turning to their resting normal shape, through their own elasticity, aspirate the 

 blood with negative pressure. Such suction-power on the part of the ventricle is, 

 however, effective only in slight degree, if at all. 



(c) With the distention of the ventricles the auriculo-ventricular 

 valves at once float upward (Fig. 26), being in part forced up by the 

 counter-stroke of the blood from the wall of the ventricle; in part they 

 are capable, by reason of their lower specific gravity, to spread out and 

 float horizontally; in part, finally, they are drawn upward by the longi- 

 tudinal muscular fibers passing from the auricles upon the valves. 



(C) The ventricles now contract and the auricles relax. In this phase 

 i. The muscular walls contract on all sides and reduce the size of 



the ventricular cavity. 

 7 



