io Dr. Young's Lecture on the Functions 
some other experiments, it was found that a moderate degree 
of pressure was capable of causing water to exude so copiously 
through the exhalant vessels of the intestines, that it passed 
through the aorta with a velocity of about two inches in a se- 
cond, although these vessels do not naturally allow any pas- 
sage to the blood : on the other hand, it sometimes happened 
that very little water would pass through such channels as 
naturally transmitted a much larger quantity of blood : a cir- 
cumstance which Dr. Hales very judiciously attributes to the 
oozing of the water into the cellular membrane surrounding 
the vessels, by means of which they were compressed, and 
their diameters lessened. On the whole, it is not improbable, 
that in some cases ^the resistance, opposed to the motion of the 
blood, may exceed that of water in a ratio somewhat greater 
than I have assigned ; but this must be in the minutest of the 
vessels, while in the larger arteries the disproportion must be 
less : so that, however we may view the subject, it appears 
to be established, that the only considerable resistance which 
the blood experiences, occurs in the extreme capillary arteries, 
of which the diameter scarcely exceeds the hundredth part of 
an inch. 
We cannot suppose that the dimensions of the sanguiferous 
system agree uniformly, in all its parts, with the measures 
which I have laid down ; but the truth of the inference is not 
affected by these variations. For example, there may perhaps 
be some arteries communicating with veins, of which the dia- 
meter exceeds the eleven hundredth of an inch ; but there are 
certainly many others which are much more minute ; and the 
blood, or its more liquid parts, passing through these more 
slowly, it must move more rapidly in the former, so that the 
