168 



PHYSIOLOGY FOR DENTAL STUDENTS. 



the clastic recoil of the arteries on the blood, the seinikuiar valves 

 are closed tightly by backward eddying currents in the arteries. 

 Their closure prevents any return of blood into the ventricles. 



The blood, having attained a certain momentum during the 

 sphygmic period, is carried on by its inertia for a fraction of a 

 second after the ventricle ceases to exert pressure on it, thus pro- 

 ducing a partially relaxed artery just beyond the semilunar 

 valves. This momentum being lost, the blood, by the pressure 

 which the stretched elastic wall of the arteries exerts on the 



Fig. 19. Diagram showing relative pressure in auricle, ventricle and aorta. 



blood, is forced back on to the semilunar valves and into the par- 

 tially relaxed base of the aorta. The blood, being thus prevent- 

 ed from returning to the heart, must continue to flow on into the 

 capillaries, and this onward flow never ceases, because the next 

 cardiac systole occurs before the arteries have ceased to exert 

 all of their recoil pressure on the blood (see also p. 173). 



After the arterial valves close, the ventricles continue to relax, 

 and the pressure within quickly falls below that which obtains 



