134 THE CIRCULATION OF THE BLOOD 



leading from the artery just beyond the compression chamber, in the above described 

 model, sounds similar to those in the arm are heard with each pulsation. While the 

 pressure is being gradually lowered from above the obliteration point, these sounds 

 will be found to become first audible as soon as a certain amount of fluid is forced 

 through the compressed area at each pulse (the systolic index), and to become louder 

 and often murmurish in quality as the decompression is proceeded with, until a pres- 

 sure is reached at which they suddenly become less intense and change in character. At 

 this moment it will be observed by watching the artery that the external pressure is 

 no longer capable of producing any flattening of the vessel between pulses. Evidently, 

 therefore, the change of sound corresponds exactly to the diastolic pressure. 



With regard to the cause, it should be clearly understood that it is the 

 systolic wave that produces it, although its occurrence and 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 pro- 

 duction of a water-hammer in the blood vessels below the compres- 

 sion cuff (Erlanger 3 ). By a water-hammer is meant the pressure changes 

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

 changes in pressure cause the walls to be thrown into vibration and so 

 produce a sound. In the taking of blood-pressure measurements, 

 as above described, when the pressure in the cuff is between the 

 systolic and diastolic, the volume of the compressed artery will in- 

 crease 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 with 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. 



