PRESSURE MEASUREMENTS IN MAN 



193 



clinical purposes. Some of these, though excellent in principle, are too com- 

 plicated for general use. The first simple and approximately accurate form 

 of apparatus was that devised by Riva-Rocci in 1896. This has been modi- 



FIG. 189. Erlanger's Sphygmomanometer, Shown with the Rubber Bag Attached to the Arm. 

 The picture is taken at the end of an experiment after the pressure in the instrument is run up again 

 to above the systolic pressure. The upper part of the cylinder shows a sphygmogram taken with 

 the instrument. (Experiment and photo by Hill and Watkins.) 



fied and improved in minor points since, but the principles of the original 

 instrument remain practically the same. 



In brief, the apparatus, figure 188, consists of an elastic tube ending in 

 a rubber bag which can be adjusted about the arm or forearm, and a mercury 

 manometer connected with this tube and also with some form of air pump 

 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, usually 

 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 

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