4 Dr . Young's Lecture on the Functions 
Dr. Hales infers, from his experiments on quadrupeds of 
different sizes, that the blood in the human arteries is sub- 
jected to a pressure, which is measured by a column of the 
height of seven feet and a half : in the veins, on the contrary, 
the pressure appears to amount to about six inches only : so 
that the force which urges the blood from the greater arteries 
through the minuter vessels into the large veins, may be con- 
sidered as equivalent to the pressure of a column of seven 
feet. 
In order to calculate the magnitude of the resistance, it is 
necessary to determine the dimensions of the arterial system, 
and the velocity of the blood which flows through it. Ac- 
cording to the measurements of Keill and others, we may 
take \ of an inch for the usual diameter of the aorta, and 
suppose each arterial trunk to be divided into two branches, 
the diameter of each being about ± of that of the trunk, (or 
more accurately 1 : 1.26 = 10 — -100567 ) , and the joint areas 
of the sections about a fourth part greater, (or 1.2586 : 1 = 
io ° 99 8 9 6 ) . This division must be continued twenty nine times, 
so that the diameter of the thirtieth segment may be only the 
eleven hundredth part of an inch, that is, nearly large enough 
to admit two globules of the blood to pass at once. The 
length of the first segment must be assumed about nine inches, 
that of the last, the twentieth of an inch only ; and supposing 
the lengths of the intermediate segments to be a series of 
mean proportionals, each of them must be about one sixth 
part shorter than the preceding, (or 1 : 1.961 = 10 — •° 777 6 ), 
the mean length of the whole forty six inches, the capacity to 
that of the first segment as 72.71 to 1, and consequently the 
weight of the blood contained in the arterial system about 9.7 
