226 THE CIRCULATION OF THE BLOOD 



nation of the systolic pressure; i.e., the maximal arterial pressure. He 

 gives for the diastolic pressure 40 to 45 mm. of mercury below the systolic 

 pressure. Other observers using the same method find a somewhat higher 

 average pressure, see figure 190, which represents a fair type of observation. 

 The form of sphygmomanometer in almost universal clinical and 

 laboratory use for determining the arterial blood-pressure of man is the 

 aneroid type of Dr. Rogers. This instrument or its various modifications 

 measures the pressure by means of the expansion of an aneroid coupled 

 with a mechanical lever and gage device. The most widely distributed 

 forms of instruments of this type are known as the Tyco and Faught. 

 These instruments use an arm belt and bag of the Riva-Rocci type. The 

 rubber bag is inclosed in a cloth belt which is conveniently wrapped 

 around the arm above the elbow. The bag contains two connections 



FIG. 190. Tracing taken with Erlanger's Sphygmomanometer. The figures indicate 

 pressure in millimeters of mercury. Systolic pressure 160; diastolic pressure, 120. (New 

 figure by Hill.) 



one of which is attached to the pressure gage, the other connected with 

 a convenient pump made of either metal of rubber. The pressure meas- 

 urement can be made directly from the oscillations of the dial as described 

 for the Erlanger or Riva-Rocci apparatus. Readings may also be ob- 

 tained by the palpation of the artery at the wrist as the pulse breaks 

 through during gradual reduction of the pressure in the arm bag. How- 

 ever, the most accurate determinations are made by the auscultatory 

 method (Goodman and Howell). A Bowles sphygmometroscope, which is 

 a stethoscope modified by a button attached to the center of the disc, is 

 attached to the arm just below the arm band at the inner angle of the 

 elbow with the button of the diaphragm directly over the brachial artery 

 near its division into the ulnar and radial. 



In operation the arm band is pumped to a pressure above that of 

 the underlying artery and then the pressure very gradually released. 

 When the external pressure just equals to or is slightly less than the 

 maximum pressure in the artery, some fluid will escape into the occluded 



