132 THE CIRCULATION OF THE BLOOD 



at which a sound first comes to be heard by listening over the artery 

 at the elbow. 



With regard to the diastolic pressure, there has been some controversy 

 as to whether it is more accurately gauged by the oscillatory or the aus- 

 cultatory method. If both methods are employed it will usually be found 

 that the oscillatory gives a higher reading 'than the auscultatory. The 

 concensus of opinion seems to be that the latter method is the more accu- 

 rate, and certainly it is the easier to apply, for with the oscillatory 

 there is often great difficulty in deciding just exactly when the maximum 

 oscillation occurs. 



The strongest evidence supporting the conclusion that the auscultatory 

 readings are more reliable than the oscillatory has been gained by ex- 

 periments with an artificial schema, consisting of a wide glass tube rep- 

 resenting the armlet, filled with Ringer's solution* and closed at both 

 ends by rubber stoppers pierced by tubes. These tubes are connected 

 with a recently excised artery, which therefore runs from end to end 

 inside the wide tube. Through tubing connected with the artery a 

 pulsatile flow of oxygenated Ringer's solution is made to flow at vary- 

 ing pressures, which are indicated by valved manometers (see page 152). 

 The pressure in the wide tube is also measured by a manometer, and 

 it is caused to vary by a suitable compressor. By comparing the be- 

 havior of the artery with the pulsating movement of a spring manom- 

 eter connected with the wide tube, under different degrees of pressure 

 inside and outside the artery, it has been observed that the maximal 

 oscillation occurs when the artery is actually somewhat flattened be- 

 tween the pulse beats; that is, it occurs at an outside pressure above 

 the diastolic pressure, at which of course the vessel should retain its 

 circular shape. When a stethoscope is applied to the tube leading 

 from the artery just beyond the wide tube, in the above described 

 model sounds similar to those in the arm are heard with each pulsa- 

 tion. While the pressure is being gradually lowered from above the 

 obliteration point, these sounds become first audible as soon as a cer- 

 tain amount of fluid is forced through the compressed area at each pulse 

 (the systolic index), and they become louder and often murmurish in 

 quality as the decompression is proceeded with, until a pressure 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 (Mac William). 



*Ringer's solution is used so that the. artery may be preserved as nearly as possible in a living 

 condition. This is important, since the elastic properties change when the arterial walls die. 



