872 PHYSIOLOGY 



escape shows that in the arteries the blood is at a high pressure, and that 

 the flow from the heart to the periphery is a pulsatory one. The same 

 lesson may be learnt by connecting a long tube with the central end of a 

 divided artery. This experiment, which was first performed by the Kev. 

 Stephen Hales, may be described in his own words : 



" In December I caused a mare to be tied down alive on her back ; she was fourteen 

 hands high, and about fourteen years of age, had a Fistula on her Withers, was neither 

 very lean, nor yet lusty : Having laid open the left crural Artery about three inches 

 from her belly, I inserted into it a brass Pipe, whose bore was one sixth of an inch in 

 diameter ; and to that, by means of another brass Pipe which was fitly adapted to it, 

 I fixed a glass Tube, of nearly the same diameter, which was nine feet in length : Then 

 untying the Ligature on the Artery, the blood rose in the Tube eight feet three inches 

 perpendicular above the level of the left Ventricle of the heart : But it did not attain 

 to its full height at once ; it rushed up about half way in an instant, and afterwards 

 gradually at each Pulse twelve, eight, six, four, two, and sometimes one inch : When 

 it was at its full height, it would rise and fall at and after each Puke two, three, or four 

 inches ; and sometimes it would fall twelve or fourteen inches, and have there for a 

 time the same Vibrations up and down at and after each Pulte. as it had, when it was 

 at its full height ; to which it would rise again, after forty or fifty Pulses." 



The method adopted by Hales of measuring the lateral pressure of 

 blood in the vessels offers very considerable drawbacks. The manipula- 

 tion of such long tubes is awk- 

 ward, and the blood which escapes 

 into the tubes very soon clots and 

 renders further observation impos- 

 sible. It is therefore customary 

 when we desire to gain an idea of 

 the average pressure in any blood- 

 vessel, especially in an artery, to 

 use a mercurial manometer for this 

 purpose. This instrument, which 

 was first applied to physiological 

 purposes by Ludwig, consists of a 

 U-tube with two vertical limbs 

 about eighteen inches in height, 

 which is half-filled with clean 

 mercury. On the surface of the 

 mercury of one limb is a float of 

 vulcanite from which a stiff fine 

 rod of straw, glass, or steel rises, 

 bearing on its upper end the 

 writing-point. This point may be 

 adjusted so as to write on the 

 blackened glazed surface of a 

 (The arrangement for imparting 

 of glazed paper is known as a 



FIG. 374. Arrangement of an apparatus for 

 taking blood- pressure tracing. 



a, artery (carotid); c, cannula; d, three- 

 way cock ; m, mercurial manometer ; 6, drum 

 covered with smoked paper ; x, tube to pres- 

 sure bottle. 



moving sheet of paper (Fig. 374). 



a continuous movement to a sheet 



kymograph.) Instead of smoking the paper a pen may be fitted to 



the end of a rod and its excursions recorded in ink on a moving band of 



