MECHANICS OF THE CIRCULATION IN THE VESSELS 113 



Measurement of the Blood-Pressure in Man. In man the blood- 

 pressure has been estimated by adjusting over an artery an instru- 

 ment known as a sphygmomanometer or sphygmometer, which, in 

 its most modern form, consists essentially of a hollow rubber pad or 

 bag containing air, and connected with a metallic pressure gauge or 

 a mercurial manometer. 



The simplest method is that devised by Riva-Rocci (Fig. 44). An 

 armlet in the form of a broad rubber bag, supported externally by 

 canvas or leather, is adjusted round the upper arm. The interior di the 

 bag is connected with a mercury manometer, and also with a strong 

 rubber bulb provided with a valve. By rhythmical compression of 

 the bulb the pressure can be raised. Between the pressure bulb and 

 the rest of the system is a thin rubber balloon, which by its distension 

 renders the changes of pressure more gradual. The finger of the 

 observer is placed over the radial artery, and the 

 pressure is raised until the pulse disappears. Then 

 the pressure is allowed to fall gradually, and the 

 manometer reading at the moment when the pulse 

 first reappears in the radial gives the maximum or 

 systolic pressure in the brachial artery. 



Instead of palpating the radial artery, a stetho- 

 scope may be placed over the brachial just below 

 the edge of the armlet, according to the method of 

 Korotkoff, by which, in addition to the systolic, the 

 minimum or diastolic pressure may be 

 determined. The pressure is raised some- 

 what above that necessary to obliterate 

 the pulse, and then allowed to fall slowly. 

 At the moment when pulsations first 

 begin to break through below the armlet, 

 a succession of sharp taps, synchronous 

 with the pulse, is heard. The tapping 



sound grows rapidly louder as the artery Fig 44 ._Ri V a-Rocci Apparatus, 

 opens up more and more, then abruptly 

 diminishes and changes its character, and 

 gradually disappears. Several phases 

 have been distinguished after the first 

 maximum, but their constancy and sig- 

 nificance are still in dispute. Every- 

 body agrees that the pressure shown by the manometer when the sound 

 is first heard is the systolic pressure. This corresponds exactly with 

 the systolic pressure as determined by palpating the radial ; and it can be 

 shown experimentally that at this point the lumen of the brachial artery 

 is actually obliterated, and not merely narrowed to such a degree as to 

 prevent the passage of the pulse wave, while still permitting the passage 

 of some blood (see Practical Exercises, p. 211). 



The diastolic pressure, according to some observers, is the pressure at 

 which the sound becomes altogether inaudible . This seems to be correct, 

 but others give it a higher value namely, the pressure at which the 

 abrupt change in the sound occurs. The sound seems to be essentially 

 due to vibrations set up in the walls of the artery and the structures 

 in contact with it when it is suddenly opened by the pulse waves, 

 although these may be intensified and otherwise modified by the 

 neighbourhood of the inflated armlet. 



The sphygmomanometer of Erlanger (Fig. 45) is arranged to obtain 



8 



a, armlet ; b, manometer tube ; 

 c, bottle containing mercury, 

 into which 6 dips; d, thin rubber 

 bulb; e, thick rubber bulb for 

 getting up pressure. 



