THE CIRCULATION 



285 



pressure when compared with the atmospheric standard, and before it 

 reaches the auricles, now expanding to receive it back, the pressure is a 

 negative quantity, that is, a small degree of suction. 



The instrument used to measure blood-pressure is called the sphyg- 

 mometer, and of this apparatus there are many forms, some of which 



FIG. 151 



Erlanger's apparatus for determination of the blood-pressure in man. The apparatus is 

 provided with a pneumatic cuff (C), which consists of an inside rubber bag and an outside leather 

 band. The whole cuff can be buckled around the arm above the elbow. The air cavity within 

 the rubber bag of the cuff communicates through a thick-walled rubber tube and a four-way 



connection, 2 =nr= with the three other essential parts of the apparatus, namely: (1) downward, 



with the valved bulb (V B), by means of which air can be forced into the cuff and can thus be 

 made to compress the arm; (2) to the left, with the mercury manometer (M), from which the 

 amount of pressure applied to the arm can be read directly in mm. of Hg.; and (3) upward, with 

 the distensible bag (B) inside the glass chamber (G). This bag, last mentioned, responds to 

 fluctuations of pressure inside the rubber bag of the arm, which are due to vibrations of the 

 arterial wall, and the tambour at the top records such vibrations on the drum (Z>). 



are practicable instruments of no little use in surgery. Their practical 

 use to the surgeon is greatly lessened by the important fact that the blood- 

 pressure in any one available artery is little indication of its degree in 

 the more vital parts of the body. It is indeed a more variable and a 

 more complicated matter than was suspected a" few years ago. It is 



