132 THE CIRCULATION OF THE BLOOD 



at which a sound first conies 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 by rubber 

 stoppers pierced by tubes, which are connected with a fresh artery, which 

 therefore runs from end to end inside the tube. Through tubing connected 

 with the artery a pulsatile flow of oxygenated Ringer's solution is made 

 to flow at varying pressures, which are indicated by valved manometers 

 (see page 152) connected with the artery tubing just beyond the com- 

 pression tube. The pressure in the latter is also measured by a manom- 

 eter, and it is caused to vary by. a suitable compressor. By comparing 

 the behavior of the artery with the pulsating movement of a spring 

 manometer connected with the compression chamber, under different 

 degrees of pressure inside and outside the artery, it has been observed 

 that the maximal oscillation occurs when the artery is actually some- 

 what flattened between 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 compression chamber, in the above de- 

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

 sation. While the pressure is being gradually lowered from above the ob- 

 literation 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 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. 



With regard to the cause of the sound, it should be clearly understood 

 that it is the systolic wave that produces it, but its occurrence and its 



