BLOOD PRESSURE 133 



character when present 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 ves- 

 sels below the compression cuff (Erlanger 3 ). By a water-hammer is 

 meant the pressure changes which are caused by suddenly stopping the 

 flow of water in a pipe. When a sudden pressure occurs in vessels with 

 elastic walls, these walls are thrown into vibration and so produce a 

 sound. In the taking of blood-pressure measurements, as above de- 

 scribed, when the pressure in the cuff is between 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 contact with the stationary blood 

 filling the uncompressed artery ~below the cuff, it will become immedi- 

 ately checked, and thus distend the arterial wall with unusual violence 

 and set it into vibration. 



