76 THE CIRCULATION OF THE BLOOD 



The Pulse 



Blood enters the arterial system intermittently and 

 leaves it at a constant rate. The arteries, therefore, at 

 each beat of the heart accommodate their capacity to an 

 increase in the volume of their contents. This they do 

 through the elasticity of their walls. Every time the 

 ventricle discharges its contents into the aorta part of the 

 kinetic energy imparted to the blood is spent in distend- 

 ing the part of the aorta nearest the heart. The distended 

 wall, in returning to its normal size, owing to its elasticity, 

 exerts a pressure upon the blood — a pressure which is trans- 

 mitted to the next segment of the aorta, which is distended 

 in consequence. In this way is caused a wave of dis- 

 tension known as the pulse- wave, which travels peri- 

 pherally at the rate of about seven metres per second. 

 The transmission of the pulse-wave is therefore a purely 

 mechanical effect, and is independent of any nervous 

 agency, except in so far as the latter may influence the 

 arterial tonus upon which the elasticity depends. The -pulse- 

 wave has nothing to do with the velocity of the blood, 

 being much faster. As it travels towards the periphery 

 the pulse-wave becomes less perceptible, the flow of blood 

 from the arterioles into the capillaries being perfectly 

 uniform. 



The nature of the pulse-wave is investigated by means 

 of the sphygmograph. This consists essentially of a spring 

 which is pressed upon the radial artery at the wrist. The 

 expansion of the artery is transmitted through the spring, 

 magnified by a system of levers, and recorded on blackened 

 paper which is moved by clockwork. Such a record is 

 shown in Fig. 18. The wave will be seen to consist of a 

 sharp upstroke and a slower downstroke. Upon the latter 

 there is invariably a smaller elevation. This is known as 

 the dicrotic wave (e), the notch preceding it {d) being called 

 the dicrotic notch. The notch is due to the fall in pressure 

 consequent upon the cessation of the outflow from the 



