11 
of the Heart and Arteries . 
resistance may in all be equal to the pressure, and the mean 
velocity may still remain such as is determined by the quan- 
tity of blood passing through the aorta. There is indeed some 
uncertainty in the measure of the globules of the blood, which 
I have made the basis of the dimensions of the minute arteries : 
and I have reason to think, that instead of of an inch, their 
greatest diameter does not exceed j bod , or even : the ge- 
neral results of the investigation are not however affected by 
this difference : it will only require us to suppose the subdi- 
visions somewhat more numerous, and the branches shorter. 
These are the principal circumstances which require to be 
considered, with respect to the simple transmission of the 
blood through the arteries into the veins, without regard to 
the alternate motions of the heart, and to the elastic and mus- 
cular powers of the vessels. I shall next examine the nature 
and velocity of the propagation of the pulse. The successive 
transmission of the pulsations of the heart, through the length 
of the arteries, is so analogous to the motion of the waves on 
the surface of water, or to that of a sound transmitted through 
the air, that the same calculations will serve for determining 
the principal affections of all these kinds of motion ; and if 
the water, which is agitated by waves, is supposed to flow at 
the same time in a continued stream, and the air which con- 
veys a sound to be carried forwards also in the form of a 
wind, the similitude will be still stronger. The coats of the 
arteries may perhaps be considered, without much inaccuracy, 
as perfectly elastic, that is, as producing a force proportional 
to the degree in which they are extended beyond their natu- 
ral dimensions ; but it is not impossible that there may be 
some bodies in nature, which differ materially from this general 
C 2 
