chap, xxi.] BLOOD PRESSURE. 227 



exerted on the tube, and the height of the column 

 of liquid that ascends in them is the measure of 

 the pressure exerted by the fluid. The first to 

 employ this method to measure the pressure of the 

 blood was Stephen Hales, rector of Faringdon. He 

 first (as early as the beginning of the eighteenth cen- 

 tury) experimented on dogs, and, later, on horses and 

 various other animals. His method was to open the 

 crural artery of the animal, and to fix into it a glass 

 tube, and then note the height to which the column of 

 blood rose in the tube. In experiments, however, to 

 determine the force of the sap in vines Hales used 

 tubes bent into a U-shape, in: the bend of which he 

 placed mercury. He noted the height to which the 

 column of mercury rose, and calculated how high a 

 column of water it represented. In his experiments 

 on blood pressure Hales noted not only 

 the height to which the column of blood 

 rose, but the time it took to attain its ^1 

 maximum, the stages by which it rose, Jlj 

 and the oscillations which it experienced 

 with the movements of the heart, and 

 other circumstances. In 1828 the bent 

 tube with mercury (Fig. 104), was em- 

 ployed by Poiseuille one end ab being in- 

 serted into the vessel of the animal, and 

 a reading then taken (by means of a scale Fi 104 

 rs, ii attached to each limb of the tube) Poiseuiiie's 

 of the difference of level of the surfaces mometei^ 

 of mercury in the two limbs hd, gc. This 

 instrument Poiseuille called a hasmadynamometer, 

 or measurer of the force of the blood. Manometer is 

 another name given to the same arrangement. 



The short limb of the bent tube was connected to 

 the artery of the animal to be experimented on 

 through the medium of a stiff elastic or a lead tube 

 with a fine extremity. A stop-cock permitted the 



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