THE CIRCULATION 229 



surround the auriculo-ventricular opening contract, while the 

 papillary muscles pull the auriculo-ventricular ring down- 

 wards and inwards through the chordae which are inserted 

 into it. 



Nevertheless the occlusion of this orifice is apt to be in- 

 complete when the right side of the heart becomes in the 

 least over-distended, giving rise to a safety valve action for 

 the right ventricle. 



The auriculo-ventricular valves are open during the whole 

 of the cardiac cycle, except during the ventricular systole 

 (Fig. 114). 



B. Semilunar Valves. Before the ventricles contract 

 these valves are closed and the various segments pressed 

 together by the high pressure of blood in the aorta. 



As the ventricles contract the pressure in them rises, until 

 the intra-ventricular pressure becomes greater than the pres- 

 sure in the arteries. Instantly the cusps of the valves are 

 thrown back and remain thus until the blood gushes out. 

 When the outflow of blood is completed, the cusps are again 

 approximated by the pressure of blood in the arteries. As 

 relaxation of the ventricles occurs, the intra-ventricular 

 pressure becomes suddenly very low, and the high pressure 

 of the blood in the arteries at once falls upon the upper 

 surfaces of the cusps which are thus forced downwards and 

 completely prevent any backflow of blood. 



The prejudicial effect of too great pressure upon these 

 cusps is obviated by the lower cusp being mounted on 

 the top of the muscular septum upon which the pressure 

 comes the other cusps shutting down upon this one 

 (Fig. 108). 



7. Flow of Blood through the Heart. The circulation of 

 blood through the heart depends upon these differences of 

 pressure in the different chambers. 



A fluid always flows from a point of high pressure to a 

 point of lower pressure. We may then consider the flow 



A. From Great Veins into Auricles. This occurs when 

 the pressure in the great veins is greater than the pressure in 

 the auricles (Fig. 114). 



The pressure in the auricles is lowest at the moment of 



