498 CIRCULATION OF BLOOD AND LYMPH. 



the systolic (or maximum) and diastolic (or minimum) pressures 

 is mdicated by the diagram in Fig. 195. The instruments that 

 have been invented for determining human blood-pressure are in 

 reaUty adapted, more or less accurately, to determine one or the 

 other or both of these pressures. No instrument has been devised 

 for determining the mean pressure, and as a matter of fact such 

 a thing as mean pressure does not exist in the large arteries, it is 

 simply an abstraction. What really occurs in these arteries is a 

 rapid swing of pressure with each heart-beat from the diastolic 

 to the systolic level, and to interpret fully our records it is impor- 

 tant to determine each of these values. The methods that have 

 been proposed for this purpose have undergone a gradual evolu- 

 tion and improvement. The determinations of systolic pressure 

 were first made, according to the principle suggested by von Basch, 

 by ascertaining the pressure that is just sufficient to occlude an 

 artery, so that a pulse wave cannot pass through. The simplest 

 form of apparatus for this purpose is the one proposed by Riva- 

 Rocci and represented in Fig. 201. Instruments of this kind are 

 known as sphygmomanometers. 



The leather or canvas band, a, is buckled snugly around the arm. On 

 the inner surface of this band there is a rubber bag which communicates with 

 the mercury manometer, d, and the pressure bulb, c. When the band is in 

 place rhythmical compressions of c will force air into the rubber bag surround- 

 ing the arm. This bag is blown up and exerts pressure upon the arm and 

 through the arm tissue upon the brachial artery. The amount of pressure 

 that is being exerted upon the arm is indicated at any moment by the mer- 

 cury manometer. The moment of obliteration of the artery is determined 

 by feeling (or recording) the pulse in the radial artery. The moment that 

 this pulse disappears, as the pressure upon the brachial is raised, indicates the 

 maximum or systolic pressure in the brachial artery. As the pressure is low- 

 ered again the pulse reappears. Several more convenient forms of this 

 instrument have been devised and are in common use among physicians. 

 In some of them a mercury manometer is employed, so that the pressure at 

 which the artery is obhterated may be read off directly in millimeters of 

 mercury, but usually on account of convenience in transportation a calibrated 

 aneroid gauge is substituted for the mercury manometer. In these latter 

 instruments the calibration given by the manufacturer should be verified by 

 direct comparison with a mercury manometer. 



For practical purposes, a great improvement was made by 

 Korotkoff in the introduction of the auscultation method by means 

 of which both systolic and diastolic pressures may be determined 

 rapidly with approximate accuracy. In this method the cuff 

 with its pneumatic bag (a) is placed round the arm above the elbow, 

 and by means of a bulb (c) or pump the pressure within the pneu- 

 matic bag is raised until the brachial artery is completely obliter- 

 ated. A stethoscope is now applied to the location of the brachial 

 artery just below the lower edge of the cuff, and by means of a 



