THE BLOODVESSELS 75 



mittent expansion and contraction of the arteries gradually 

 becomes less marked at a distance from the aorta, and dies out 

 at the arterioles. 



Attention has previously (p. 65) been drawn to the fact 

 that the elastic properties of the arterial wall, together with the 

 peripheral resistance in the smallest bloodvessels, convert the 

 intermittent flow started by the heart into the continuous 

 stream in the capillaries and veins. In seeking for the cause 

 of the disappearance of the pulse, we find it similarly in the elastic 

 property of the arterial walls. In virtue of this property each 

 inch of the arteries is engaged, by means of its sudden distension 

 after each heart-beat and its more gradual elastic recoil before 

 the next, in sheltering the capillaries from the effect of that beat. 

 The oscillations of pressure which give rise to the pulse are, so 

 to say, ' damped ' by the elastic arterial walls, or in other words 

 converted into a steady pressure, a fraction of the pulse being 

 thus actually destroyed by each inch of the arteries. When 

 all the fractions thus destroyed are added together, we can readily 

 understand why the initial ■ jerk,' to which the pulse is due, has 

 entirely disappeared just before it would otherwise have reached 

 the capillaries. If the arterioles dilate considerably, when, in 

 fact, less elastic recoil of their walls is called into play by the 

 lessened peripheral resistance, it may be possible for the ' throb ' 

 to pass not only through the arterioles but also the capillaries, 

 and appear in the veins ; in this way a venous pulse may be 

 produced. An example of this has been given on p. 67. 



The intermittent expansion of the arteries, called the pulse, 

 travels from the aorta to the periphery, and produces a wave 

 in the arterial system which is spoken of as the pulse-wave. 

 From what has been said it is evident that the height of this 

 wave is greatest nearest the heart, and falls to zero at the capil- 

 laries. The wave travels with considerable velocity, from 4J to 

 9 metres (15 to 30 feet) per second. This may easily be deter- 

 mined by noting the interval between the commencing succes- 

 sive rises of two levers, resting consecutively on the wall of an 

 artery, at a measured distance apart. The length of the pulse-, 

 wave is also considerable — viz., about 5J metres (18 feet). This 

 is arrived at by noting the time each single pulsation, travelling 

 with the previously determined velocity, takes to pass completely 

 under any one lever. Putting these data together, it is evident 

 that the beginning of each pulse-wave is lost in the arterioles 

 before its end has left the aorta. 



No mental confusion should exist as to the difference, and the 

 causes of that difference, between the velocity of the pulse-wave 

 and the velocity of the onward flow of the blood. The factors 

 which give rise to them are quite distinct. The pulse-wave runs 



