LECT. XVII. PRESSURE ON THE BLOOD. 325 



constant throughout the experiment. The difference in the 

 level of the two columns of mercury, indicates the pressure 

 exercised by the blood against the corresponding section of 

 the wall of the vessel into which the tube of the apparatus 

 is introduced. Poiseuille made a great number of experi- 

 ments upon the arteries of different animals, and upon 

 various arteries of the same animal ; and the most important 

 fact which he demonstrated is, that the pressure of the blood 

 in the arteries is the same, whatever be the part of the 

 arterial system, the diameter of the vessel, its distance from 

 the heart, and the position of the branch experimented on, 

 in relation to the trunk from which it is derived. Thus, 

 Poiseuille found that the pressure obtained, by applying the 

 heemo-dynamometer to the carotid artery of a dog, at a dis- 

 tance of 180 millimetres from the heart, is the same as that 

 of the aorta at 370 millimetres : the diameter of the aorta 

 being 9 millimetres, and that of the carotid 4 millimetres. 

 In both cases the pressure was measured by a column of 

 mercury 84 millimetres in height. This experiment, made 

 on a horse, gave the same result, when performed on two 

 arteries, the diameter of one of which was five times greater 

 than that of the other: the pressure made by both was equal 

 to that of a column of mercury of 146 millimetres. It is 

 curious to observe, that the pressure, in different animals, 

 has no relation to their weight. 



This result, which it is right to have had demonstrated 

 by experiment, ought not, however, to astonish us when we 

 reflect that it is a necessary consequence of the principle of 

 the equality of pressure of liquids. The impulse given by 

 the column of blood expelled from the left ventricle, to that 

 contained in the aorta, is immediately propagated equally 

 to the whole mass of blood, filling both the large and small 

 arteries. 



It is this impulse, which takes place at the moment when 

 the ventricle propels the blood into an artery and into alj 



