214 



THE CIRCULATION OF THE BLOOD 



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

 duced 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 

 small percentage of the pressure, an error which may be ignored for all 

 comparative purposes. 



Erlanger has perfected a form of sphygmomanometer which contains a 

 very ingenious and compactly arranged recording device, figure 189. This 

 instrument has a mercury manometer from which the pressures are read off 

 directly. On a side limb of the manometer there is a rubber bag enclosed 

 in a glass bell. The cavity of the bell outside of the rubber bag is connected 



