THE . BLOODFLOW IN THE ARTERIES 205 



put, for, it might be said, why should there be such a sudden fall in 

 pressure near the heart, whereas toward the periphery, as we have seen, 

 the pressure between the heartbeats tends to be maintained on account 

 of the elastic recoil of the stretched arterial walls. The explanation 

 usually given is that the sudden cessation of outflow of blood from the 

 ventricle at the end of the sphygmic period causes a negative pressure 

 to be produced in the blood at the beginning of the aorta, thus tending 

 to cause a reflux of blood towards the heart, the effect of which is (1) to 

 bulge the closed valves, and (2) to produce the reflected dicrotic wave. 

 If, while fluid is flowing under pressure along a tube, the flow is sud- 

 denly arrested by turning a stopcock, it is possible by the use of manom- 

 eters to show that a negative wave is set up immediately beyond the 

 stopcock, and that this negative wave travels along the tube at a rate 

 depending on the elasticity of its walls. 



Causes for Disappearance of the Pulse in the Veins 



The disappearance of the pulse in the capillaries and its consequent 

 absence in the veins we have already seen to be owing to the combined 

 influence of the elasticity of the vessel walls and the peripheral resist- 

 ance. On account of these two factors the pressure conveyed to the 

 blood during systole is stored up to be released during diastole by the 

 recoil of the stretched vessels. Sometimes, however, the pulse gets 

 through to the veins, either because the elasticity of the vessels is not so 

 marked, or because the peripheral resistance has been lowered (vaso- 

 dilatation). In patients with hardened arteries, or in normal individu- 

 als after taking nitrite, which dilates the peripheral arterioles, a pulse 

 may come through at the periphery and appear in the veins. This may 

 be called the peripheral venous pulse, and it is to be carefully distin- 

 guished from the central venous pulse observed in the large veins, as 

 at the root of the neck, before any valves have intervened to block the 

 transmission of the auricular pressure wave back into the column of 

 blood in the veins. If a pulse is seen in a large vein and there is 

 doubt as to whether it is peripheral or central in origin, this doubt can 

 be immediately removed by locally constricting the vein; if the pulse 

 is peripheral, it will disappear on the heart side of the constriction; if 

 it is central, on the side away from the heart. 



