VELOCITY AND PRESSURE OF BLOOD-FLOW. 501 



reality 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 

 j)lace 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. Among other sources of error involved in 

 this method it is to be remembered that the tactile sensibility is not sufficiently 

 delicate to detect a minimal pulse in the artery. Other methods of determin- 

 ing the systolic pressure (see below) indicate, as a matter of fact, that the 

 pulse continues some time after an individual of average tactile sensibility is 

 unable to detect it. 



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 

 needle valve the pressure on the artery is allowed to drop slowly. 

 The moment that the pressure falls to a point at which a pulse 

 wave can break through the constricted area a distinct sound 

 will be heard in the stethoscope. If the mercury manometer is 

 read at this point, it will give the value for the systolic pressure. 

 As the outside pressure continues to fall the sound becomes fuller, 



