18 



THE BLOOD 



The following account of his manometer, and 

 the picture of it, are given in his thesis : — 



"Take a glass tube, having a horizontal arm 

 AB, a vertical descending arm BC, and a third 

 ascending arm DE, and curved to a quarter of a 

 circle at B, and to half a circle at CD. Suppose 

 that we put mercury in the part GCDH, the tube 

 being vertical, the levels G and H of the mercury 

 will be the same in the two arms. If the blood be 

 passed into ABG through the orifice A, connected 

 with an artery, it will press on the surface of the 

 mercury at G : the metal will be pressed down in 

 the arm BC from G to K, for example, when it will 

 rise in the arm DE to I. It is evident, from the 

 laws of hydrostatics, that the total force with which 

 the blood moves in the artery will be measured by 

 the weight of a cylinder of mercury, whose base is a 

 circle of the diameter of the artery, and whose height 

 is the difference IK between the two levels of the 

 mercury ; deduction being made, of course, of the 

 height of the little column of mercury balancing the 

 column of blood BK." 



He found also, by experiments, that the coagula- 

 tion of the blood in the tube could be prevented by 

 filling the part ABG of the tube with a saturated 

 solution of sodium carbonate. The tube, thus pre- 

 pared, was connected with the artery by a fine 

 cannula, exactly fitting the artery. With this 

 instrument, Poiseuille was able to obtain results 

 far more accurate than those of Hales, and to 

 observe the diverse influences of the respiratory 

 movements on the blood-pressure. He sums up 

 his results in these words : — 



