224 



THE CIRCULATION OF THE BLOOD 



used for inflating the tube about the arm and thus exerting pressure upon 

 its blood-vessels. The elastic tube is covered by some inelastic tissue, usu- 

 ally a leather cuff, in order that the inflation of the bag may cause the full 

 increase of pressure to be exerted upon the encased arm. By inflating the 

 bag until the pulse at the wrist just disappears, and reading the height of 

 the column of mercury in the manometer, the maximum or systolic 

 pressure is obtained in millimeters of mercury. If now the pressure on 

 the arm is reduced until the widest oscillations of the mercury column 

 are obtained, the lowest position of the mercury meniscus represents the 

 diastolic pressure. 



The apparatus depends on the principle that an external pressure just 

 equal to the maximal pressure within an artery will hold the vessel in the 

 collapsed condition, a fact that has been proven for vessels that are exposed. 

 An external pressure that will just equal the minimal or diastolic pressure 

 will cause a complete collapse of a vessel during diastole and will allow a 

 complete expansion of an artery to its maximal limits during the systolic 

 period of pressure. In other words, the mercury of the manometer will 

 oscillate to its maximal. If the pressure is reduced to a still lower point, it 



FIG. 188. Riva-Rocci Apparatus (schematic) for Determining Blood Pressure in Man. 



will not be sufficient to compress the artery completely, and the mercury 

 oscillations will again become smaller. In applying the instrument to the 

 brachial artery, one must, of course, deal with a vessel deeply buried in mus- 

 cular and other tissues. These latter tissues probably consume a certain 



