454 CIRCULATION OF BLOOD AND LYMPH. 



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. 188. Figure of the Riva-Rocci apparatus (Sahli) : 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, b, to meas- 

 sure the amount of pressure. 



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- 



Fig. 189. 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; 6 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. 



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. 



To determine the diastolic pressure is more difficult and requires some- 

 what more apparatus. The principle employed was first suggested by Marey 

 and first practically applied by Mosso.* The method consists in recording 



* "Archives italiennes de biologic," 23, 177, 1895. 



