THE PUMPING ACTION OF THE HEART 155 



of time longer than the auricular wave which causes them, because of 

 the elastic nature of the ventricular wall, so that the valve flaps close 

 with perfect apposition not merely at their edges but also for a con- 

 siderable distance along their upper surfaces. 



When ventricular systole starts, the only effect of the high pressure 

 which is brought suddenly to bear on the under surfaces of the already 

 closed valves is to cause them to vibrate and to bulge into the auricles, 

 being meanwhile anchored down and prevented from flapping into the 

 auricle by the chordae tendineae. There is reason to believe that the 

 musculi papillares to which these are attached begin to contract at the 

 very outset of ventricular systole indeed slightly to precede it (see 

 page 274), and thus keep the chordae taut. As systole continues the 

 contraction of these muscles becomes more and more pronounced, and the 

 resulting tightening of the chordae serves to draw down the valve flaps, 

 so that progressively larger proportions of their upper aspects tend to 

 become opposed. Meanwhile the auriculoventricular orifice is also be- 

 coming narrowed down on account of the contraction of the musculature 

 of the auriculoventricular groove. 



Semilunar Valves 



The mechanism involved in the operation of the semilunar valves is 

 somewhat different. It has been shown that, when fluid is flowing in a 

 tube, the pressure and velocity are not equal in the axial and peripheral 

 parts of the stream. In the axis the velocity is greater than in the layers 

 of fluid next to the walls, but the pressure is less. This can be seen by 

 observing through a wide glass tube the flow of water in which are sus- 

 pended lycopodium spores. By placing within the wide tube small bent 

 tubes so arranged that one open end lies near the periphery and the other 

 near the axis, the differences in pressure between the axial and peripheral 

 streams can be seen to cause the fluid to flow in the narrow tubes from pe- 

 riphery to axis (centripetal eddies). 



If the tube should suddenly expand the eddy currents become still 

 more pronounced just where the wider portion starts. In the conditions 

 obtaining at the beginning of the large arteries of the heart, the orifice into 

 the ventricles being constricted, a centripetal vortex must be set up, tend- 

 ing to throw the valve flaps into a closed position, which, however, is pre- 

 vented by the blood rushing between them from the ventricles. They thus 

 take up a mid-position and vibrate in the stream. "When the efflux from 

 the ventricle stops at the end of systole, the reflux, lasting for a moment 

 longer and being now unopposed, immediately closes the valves, in which 

 position they are then maintained by the greater pressure on their aortic 

 surfaces. 



