8 
Dr. Young’s Lecture on the Functions 
water, which is more than twice as much as appeared in the 
larger tube. Hence there can be no doubt that the resistance 
of the internal surface of the arteries to the motion of the 
blood must be much greater than would be found in the case of 
water : and supposing it about four times as great, instead of 
20 inches, we shall have 80, for the measure of a column of 
which the pressure is capable of forcing the blood, in its na- 
tural course, through the smaller arteries and veins, which 
agrees very well with Hales’s estimate. 
This determination of the probable dimensions of the arte- 
rial system, and of the resistances occasioned by its different 
parts, is in some few respects arbitrary, at the same time that 
it cannot be materially altered without altering either the 
whole quantity of blood contained in the body, the diameters 
of the smallest capillary vessels, the mean number of bifurca- 
tions, or the magnitude of the resistance, all of which are here 
assumed nearly as they have been laid down by former ob- 
servers : the estimation of the length of the successive seg- 
ments only is made in such a manner, as to reconcile these 
data with each other, by means of the experiments and cal- 
culations relating to the friction of fluids in pipes. The effect 
of curvature in increasing the resistance has been hitherto 
neglected ; it can be only sensible in the larger vessels ; and 
supposing the flexures of these to be equivalent to the cir- 
cumferences of two circles, each two inches in diameter, the ra- 
dius q being 1 , we have r = -- - — v q 8 — .000004,5 x 720 x 64, 
— .207, or about one fifth of an inch, for the additional resist- 
ance arising from this cause in the case of water, or four fifths 
for blood, which is a very inconsiderable part of the whole. 
