1 66 THE CIRCULATION. 



necessary merely to multiply tlie area of a transverse sec- 

 tion of a vessel by the height of the column of mercury 

 which is already known to be supported by the force of 

 the blood in any part of the arterial system. The weight 

 of a column of mercury of the dimensions thus found, will 

 represent the pressure exerted by the column of blood. 

 And assuming that the mean of the greatest and least 

 height of the column of mercury found, by experiments 

 on different animals, to be supported by the force of the 

 blood in them, is equivalent to the height of the column 

 which the force of the blood in the human aorta would 

 support, he calculated that about 4lbs. 4oz. avoirdupois 

 would indicate the static force with which the blood is 

 impelled into the human aorta. By the same calculation, 

 he estimated the force of the circulation in the aorta of the 

 mare to be about 1 1 Ibs. 9 oz. avoirdupois : and that in the 

 radial artery at the human wrist only 4 drs. We have 

 already seen that the muscular force of the right ventricle 

 is equal to only half that of the left, consequently, if 

 Poiseuille's estimate of the latter be correct, the force with 

 which the blood is propelled into the lungs will only be 

 equal to 2 Ibs. 2 oz. avoirdupois. 



The amounts above stated indicate the pressure exerted 

 by the blood at the several parts of the arterial system at 

 the time of the ventricular contraction. During the dila- 

 tation, this pressure is somewhat diminished. Hales 

 observed, that the column of blood in the tube inserted 

 into an artery, falls an inch, or rather more, after each 

 pulse; Ludwig has observed the same, and recorded it 

 more minutely. The pressure is also influenced by the 

 various circumstances which affect the action of the heart ; 

 the diminution or increase of the pressure being pro- 

 portioned to the weaker or stronger action of this organ. 

 Valentin observed that, on increasing the amount of blood 

 by the injection of a fresh quantity into it, the pressure in 

 the vessels was also increased, while a contrary effect 

 ensued on diminishing the quantity of blood. 



