BLOOD PRESSURE 133 



It should be clearly understood that it is the systolic wave that pro- 

 duces the sound, but its occurrence and its character are dependent 

 upon the intra-arterial pressure existing during the diastolic phase. 

 The cause of the sound has been shown to depend on the production of 

 a water-hammer in the blood vessels below the compression cuff (Er- 

 langer 3 ). By a water-hammer is meant the pressure changes which 

 are caused by suddenly stopping the flow of water in a tube. When a 

 sudden pressure occurs in tubes with elastic walls, these walls are thrown 

 into vibration and so produce a sound. In the taking of blood-pressure 1 

 measurements, as above described, when the pressure in the cuff is be- 

 tween systolic and diastolic, the volume of the compressed artery will 

 increase abruptly with each heartbeat and thus permit a considerable 

 volume of swift-flowing blood to enter the rest of the artery underneath 

 the cuff. When this quickly moving column of blood comes into con- 

 tact w T ith the stationary blood filling the uncompressed artery below the 

 cuff, it will become immediately checked, and thus distend the arterial 

 wall with unusual violence and set it into vibration. 



