9 
of the Heart and Arteries. 
It might be questioned whether the experiments, which I 
have made, with tubes -A.- of an inch in diameter, are suffi- 
cient for determining, with accuracy, the degree in which the 
resistance would be increased in tubes, of which the diameter 
is only one sixth part as great ; and it may be doubted whe- 
ther the analogy, derived from these experiments, can be 
safely employed as a ground for asserting, that so large a 
portion of the arterial pressure is employed in overcoming the 
resistance of the very minute arteries. But it must be remem- 
bered, that these experiments are at least conclusive with re- 
spect to the arteries larger than the tubes employed in them, 
and even those which are a little smaller ; so that the remain- 
ing pressure, as observed in experiments, can only be em- 
ployed in overcoming the resistance of the minuter arteries 
and veins, and these observations tend therefore immediately 
to confirm the analogy drawn from the experiments on the 
motion of water. It might indeed be asserted, that the vis- 
cidity of the blood exceeds that of water in a much greater 
ratio than that which is here assigned ; but this is rendered 
improbable by some experiments of Hales, in which, when 
the intestines were laid open, on the side opposite to the me- 
sentery, so that many of the smaller arteries were divided, 
the quantity of warm water which passed through them with 
an equal pressure, was only about twelve times as great as 
that of the blood which flows through them in their natural 
state ; and it is probable, that at least three or four times as 
much of any fluid must have passed through them in their 
divided, as in their entire state, unless we suppose that the 
coats of the divided vessels, like many other muscular parts, 
are capable of being contracted by the contact of water. In 
MDCCCIX. C 
