FORCE OF BLOOD IX ARTERIES. 153 



pressure equal to the force by which it is moved in the 

 arteries ; and the mercury will, in consequence, descend in 

 this branch, and ascend in the other. The depth to which 

 it sinks in the one branch, added to the height to which it 

 rises in the other, will give the whole height of the column 

 of mercury which balances the pressure exerted by the 

 blood ; the weight of the blood, which takes the place of 

 the mercury in the descending branch, and which is more 

 than ten times less than the same quantity of quicksilver, 

 being subtracted. Poiseuille thus calculated the force 

 with which the blood moves in an artery, according to the 

 laws of hydrostatics, from the diameter of the artery, and 

 the height of the column of quicksilver ; that is to say, 

 from the weight of a column of mercury, whose base is a 

 circle of the same diameter as the artery, and whose height 

 is equal to the difference in the levels of the mercury in 

 the two branches of the instrument. He found the blood's 

 pressure equal in all the arteries examined ; difference in 

 size, and distance from the heart being unattended by any 

 corresponding difference of force in the circulation. The 

 height of the column of mercury displaced by the blood 

 was the same in all the arteries of the same animal. The 

 correctness of these views having been questioned, Poi- 

 seuille has recently repeated his observations, and obtained 

 the same results. 



From the mean result of several observations on horses 

 and dogs, te calculated that the force with which the 

 blood is moved in any large artery, is capable of support- 

 ing a column of mercury six inches and one and a half 

 lines in height, or a column of water seven feet one line in 

 height. With these results, the more recent observations 

 of other experimenters closely accord. Poiseuille's experi- 

 ments having thus shown to him that the force of the 

 blood's motion is the same in the most different arteries, 

 he concluded that, to measure the amount of the blood's 

 pressure in any artery of which the calibre is known, it is 



