VELOCITY AND PRESSURE OF BLOOD-FLOW. 467 



is very simple: it consists in determining the amount of pressure 

 necessary to completely obliterate the artery, that is, to prevent 

 a pulse from passing through the region under compression. This 

 principle was used originally by von Basch, but its application has 

 been made perhaps most' successfully in the simple apparatus 

 suggested by Riva-Rocci, which is adapted especially for measure- 

 ments of pressure in the brachial artery. One form of this instru- 

 ment is represented in Fig. 191. 



Fig. 191 Figure of the Riva-Rocci apparatus (Sahlf) : a, The leather collar with 

 inside rubber bag to go on the arm ; c, the bulb for blowing up the rubber bag and thus 

 compressing the artery; d, the manometer dipping into the reservoir of mercury, 6, to meas- 

 sure the amount of pressure. 



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- 



Fig. 192. Schema to illustrate the fact that when the pressure upon the outside of the 

 artery is equal to the diastolic pressure the pulse wave will cause a maximal expansion of 

 the artery : a represents the normal artery distended by diastolic blood-pressure ; the dotted 

 lines indicate the additional expansion caused by the pulse wave; b represents the artery 

 when compressed by an outside pressure equal to the diastolic pressure within; the artery 

 then takes the size of an empty artery kept patent by the rigidity of its walls. The pulse 

 wave, on reaching this section, finds a relaxed wall and causes, therefore, a maximum 

 extension. 



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- 



