THE PUMPING ACTION OP THE HEART 



155 



delay causing the reflected wave to become more pronounced. At the 

 same time the muscular fibers in the valve flaps (Kiirschner's fibers) 

 contract and make the flaps shorter, the total effect of the two factors 

 being that the valve takes up a position nearer that of closure. When 

 presystole suddenly stops, the reflexed waves will persist for an instant 

 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 opposition 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 



Fig. 36. Diagram to show the positions of the cardiac valves: 1, during diastole; 2, during 

 the presphygmic period; 3, during the sphygmic period. 



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

 page 263), 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. The different velocities 



