78 THE CIRCULATION OF THE BLOOD 



Clinical Methods 



For clinical purposes the sphygmomanometer is employed. 

 The Riva-Rocci pattern, which is the one most commonly 

 used, consists of a canvas band which is tied round the 

 upper arm. On the inner side of the band is a rubber bag 

 which, on being inflated with air, compresses the arm. The 

 air inside the bag communicates with a pump, with a 

 mercurial or spring manometer, and through a valve with 

 the external air. Air is pumped in until the radial pulse 

 can no longer be felt. The pressure is then gradually 

 released by opening the valve, and the reading of the 

 manometer noted at which the pulse just becomes per- 

 ceptible. This gives the systolic pressure. 



By an adaptation of this instrument it is possible to 

 estimate the diastohc as well as the systohc pressure. 

 When the pressure as recorded by the manometer is such 

 that the pulse is barely perceptible, it means that the 

 brachial artery is completely compressed except at systole, 

 when the pressure within the artery is just sufficient to 

 overcome the pressure tending to obliterate the artery. 

 As the external pressure is gradually reduced the systohc 

 pressure comes through more easily, the artery being still 

 compressed at diastole. It is obvious that when the external 

 pressure is just sufficient to compress the artery at diastole, 

 the extra pressure produced in the artery by systole will 

 exert its maximum dilating effect. If the oscillations of 

 the manometer be recorded on a writing surface, as in 

 Gibson's apparatus, the point at which the excursion of 

 the lever is greatest marks the diastohc pressure. 



The diastohc pressure can also be estimated by hstening 

 through a stethoscope placed over the brachial artery at 

 the elbow. Beginning with complete obhteration of the 

 pulse, as the pressure is released faint sounds are heard 

 when the systohc wave begins to come through. With further 

 lowering of the pressure a stage is reached at which the 

 sounds suddenly become louder and sharper. From this 



