t its return to the heart,
it must therefore move in the direction in which it finds least
resistance.

If it were not for this muscular power of the arteries, the force of
the heart would not alone be able to propel the blood to the extreme
parts of the body, and overcome the different kinds of resistance it
has to encounter. Among the causes that lessen the velocity of the
blood, may be mentioned the increasing area of the artery; for it was
before observed, that the sum of the cavities of the branches from
any trunk exceeded the cavity of the trunk: and from the principles
of hydrostatics, the velocities of fluids, propelled by the same
force, in tubes of different diameters, are inversely as the squares
of the diameters, so that in a tube of double the diameter, the
velocity will only be one fourth; in one of the triple, only one
ninth: and since the arteries may be looked upon as conical, it is
evident that the velocity of the blood must be diminished from this
cause.

The curvilinear course of the arteries likewise gives considerable
resistance; for at every bending the blood loses part of its momentum
against the sides; and this loss is evidently proportioned to the
magnitude of the angle, at which the branch goes off. Convolutions
are frequently made, in order to diminish the force of the blood in
particular organs; this is especially the case with the carotid
artery before it enters the brain.

The angles which the ramifications of the arteries make, are greater
or more obtuse nearer the heart, and more acute as the distance
increases; by which means the velocity of the blood is rendered more
equal in different parts.

The anastomosing or union of different branches of arteries, likewise
retards the velocity of the blood, the particles of which, from
different vessels, impinging, disturb each other's motion, and
produce a compound force, in which there is always a loss of
velocity: and it is evident, from the composition of forces, that
this loss must be proportioned to the obliquity of the angle at which
the vessels unite.

The adhesion of the blood to the sides of the vessels, likewise
causes a loss of velocity in the minuter branches, which may be owing
to a chemical affinity: the viscidity or imperfect fluidity of the
blood is another retarding cause. All these causes united, would
render it impossible for the heart to propel the blood with the
velocity with which it moves in the very minute branche