156 



THE CIRCULATION OP THE BLOOD 



can be demonstrated by observing the flow through a wide tube of water in 

 which are suspended lycopodium spores. By placing in the tube small 

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

 the other near the center, it can be seen that the differences in pressure 

 are such as to cause the fluid to flow from periphery to axis (centripetal 

 eddies). 



If the bent tubes are used to study the conditions of flow in a tube which 

 suddenly becomes wider, it will be found that where the wide portion 

 starts centripetal eddies are set up, which tend to carry the spores into 

 the axis of the stream, where their velocity is greatly increased. Now 

 these are the conditions obtaining at the beginning of the large arteries 



S.a.-D.v. 



D.a.-S.v. 



Fig. 37. Diagram showing the position of the cardiac chambers and valves during presystole 

 (S.a. -D.v.) and during the sphygmic period. (From Landois.) 



of the heart, the orifice into the ventricles being constricted, while at 

 the sinus valsalvae the vessels are dilated. A centripetal vortex must be 

 set up in the sinus, tending to throw the valve flaps into a closed posi- 

 tion, which, however is prevented 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 sys- 

 tole, 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 upper surfaces. 



The position of the valves relative to the events of the cardiac cycle is 

 shown in Figs. 36 and 37. 



