APPARATUS FOR PHYSIOLOGICAL USE. 233 



arrest the action of the heart, I find that the capillary 

 blood-stream continues for several seconds, notwithstanding 

 that no more blood is pumped along the arteries, and thereby 

 learn that the circulation through the living tissues is only 

 mediately dependent on the action of the heart. 



"What then is the immediate cause of the motion of 

 blood in the capillaries'? The cause is simply the differ- 

 ence of internal pressure between the arteries and veins. 

 By this, I mean that if it were possible to connect with 

 one of my arteries (say the radial) a pressure gauge by 

 which I could determine the difference between the pressure 

 exerted by the liquid on its internal surface and that of 

 the atmosphere, and to connect a similar gauge with one of 

 the veins by which blood is brought back again from my 

 hand, I should find that there was a difference between the 

 two gauges of about four inches of mercury in other 

 words, that in the arteries the pressure would exceed that 

 in the veins by something like two pounds on the square inch. 

 That is the simple reason why the blood flows out of the 

 radial artery into the veins which come back from my hand. 



The first person to demonstrate this fundamental 

 fact in physiology was a beneficent clergyman of the last 

 century, Dr. Hales, who made experiments, in which, 

 having inserted a long vertical tube into an artery of a 

 living animal, the other end of which was open, he found 

 that the tube filled to a height of several feet, the column 

 of . blood dancing up and down at each stroke of the 

 heart pump, whereas, when he inserted a similar tube into 

 a vein, the blood either did not rise at all, or rose only a 

 few inches. 



For a long time physiologists were content to leave these 

 facts just as Dr. Hales first discovered them. It was not 

 until late in the present century that Poiseuille gave them 

 precision by substituting for Dr. Hales' tubes the mercurial 

 manometer, i.e., a glass tube bent into the form of a (J, 

 supported vertically on a suitable stand, and half filled 

 with mercury. If one limb of such a tube is in commu- 

 nication with an artery, in such a way that the same 

 pressure is exercised on the surface of the mercury con- 

 tained in it as on the internal surface of the artery, while 

 the surface of the column of mercury in the other limb 

 remains open, the difference between the pressure in the 



