of the Heart and Arteries. 3 
When we consider the blood vessels as tubes of invariable 
dimensions, we may suppose, in order to determine the velo- 
city of the blood in their different parts, and the resistances 
opposed to its motion, that this motion is nearly uniform, since 
the alternations, arising from the pulsation of the heart, do 
not materially affect the calculation, especially as they are 
much less sensible in the smaller vessels than in the larger 
ones, and the principal part of the resistance arises from these 
small vessels. We are to consider the blood in the arteries 
as subjected to a certain pressure, by means of which it is 
forced into the veins, where the tension is much less consi- 
derable ; and this pressure, originating from the contractions 
of the heart, and continued by the tension of the arteries, is 
almost entirely employed in overcoming the friction of the 
vessels : for the force required to overcome the inertia of the 
blood is so inconsiderable, that it may, without impropriety, 
be wholly neglected. We must therefore inquire, what the 
magnitude of this pressure is, and what degree of resistance 
we can suppose to arise from the friction of the internal sur- 
face of the blood vessels, or from any other causes of retarda- 
tion. The magnitude of the pressure has been ascertained by 
Hales's most interesting experiments on a variety of animals, 
and may thence be estimated with sufficient accuracy for the 
human body ; and for determining the magnitude of the re- 
sistance, I shall employ the theorems which I have deduced 
from my own experiments on very minute tubes, compared 
with those which had been made by former observers under 
different circumstances ; together with some comparative ex- 
periments on the motion of water and of other fluids in the 
same tubes. 
B 2 
