The Measurement of Arterial Pressure in Man. 513 



Suppose one raises the pressure in the armlet on the upper arm to 100 mm. 

 and so develops at the elbow the loud characteristic murmur. If one then 

 raise the pressure in another armlet placed below the auscultating tambour, 

 to well above the known systolic pressure in the artery, this murmur 

 disappears but a sound synchronous with each pulse beat appears in its place. 

 This is the dull sound due to the sudden tension of the arterial wall, a sound 

 independent altogether of those vibrations which are set up in the arterial 

 wall by that inrush and outrush of the blood which is synchronous with the 

 crest of the systolic wave. On lowering the armlet pressure to the level at 

 which the outflow of blood regains sufficient velocity the characteristic 

 murmur returns. We conclude, therefore, that stoppage of the blood flow by 

 the lower armlet while the pressure in the upper armlet ranges up to 

 systolic pressure, cannot prevent the occurrence of sounds, though it leaves 

 their quality changed. The sounds due to vibrations set up by the 

 sudden in and outrush of blood disappear ; the sounds due to the periodic 

 sudden tension of the arterial walls persist. 



As we have said before, in an artery placed on bone or glass and compressed 

 with an armlet (the armlet not embracing pulsing tissues), or with the bag of 

 the pocket sphygmometer, the pulse is obliterated by pressures below diastolic 

 pressure. 



By the reinforcement of the pulse in the vessels of the tissues which are 

 enclosed by the armlet or bag, used in the ordinary way, these critical 

 pressures are successfully passed, and the normal process of arterial deforma- 

 tion proceeds at the proper level. So accurate systolic blood-pressure 

 measurements, both auditory, tactile, and visual, become possible. On this 

 mechanism — the conserving effect of the tissue vessels on the pulse — depends 

 the accuracy of the auditory method of estimating the diastolic pressure. 

 Without this mechanism the diastolic level would be too low. Thus we have 

 found it so when the bag of the pocket sphygmometer, or armlet, used so as 

 not to embrace pulsing tissues (as described in a previous communication, 

 loc. cit.) is applied to the aberrant radial. But when the armlet is applied 

 round the arm then the diastolic auditory index, as heard in the aberrant 

 radial, comes much closer to the truth. 



It has been noted by MacWilliam and Melvin, and others, that sounds can 

 be produced which are audible at the brachial artery at the elbow when finger 

 pressure is applied to the brachial artery in the arm. Hill, McQueen, and 

 Flack {loc. cit.) have shown that finger pressure applied discretely to any 

 artery, brachial or radial, deforms the artery in a precisely similar manner as 

 does the bag, or armlet (used with the box so as not to embrace pulsino- 

 tissues), when applied to the aberrant radial. The pulse is damped out below 



