s of the
arteries, if these arteries were not endowed with a living muscular
power like the heart, by which they contract and propel their
contents.

In the veins, the motion of the blood is occasioned partly by the vis
a tergo, and partly by the contraction of the neighbouring muscles,
which press upon the veins; and these veins being furnished with
valves, the return of the blood towards the arteries is prevented; it
must therefore move towards the heart.

That the contraction of the muscles of the body tends very much to
promote the circulation of the blood, is evident, from the increase
of the circulation from exercise, and likewise from the languid
motion of the blood in sedentary persons, and those given to
indolence. Hence we may account for the different diseases to which
such persons are subject, and know how to apply the proper remedies.
Hence likewise, we see the reason why rest is so absolutely necessary
in acute and inflammatory diseases, where the momentum of the blood
is already too great.

It has been doubted by anatomists, whether the veins were possessed
with muscular power; but this seems now to be confirmed. Haller found
the vena cava near the heart to contract on the application of
stimulants, though he could see no muscular fibres; these, however,
have been discovered by succeeding anatomists.

The magnitude of the veins is always greater than that of the
corresponding arteries; hence the velocity of the blood must be less
in the veins; and hence likewise we may account for their want of
pulsation; for the action of the heart upon the arteries is at first
very great; but as we recede from the heart, this effect becomes less
perceptible; the arterial tube increases both in size and
muscularity, in proportion to its distance from the source of
circulation. The powers of the heart are spent in overcoming the
different resistances which I have noticed, before the blood enters
the veins; hence the blood will flow uniformly in these last.

The blood is subject in the veins to retarding causes, similar to
those which operate in the arteries, but perhaps not in an equal
degree; for the flexures are less frequent in the veins than in the
arteries. As the capacity of the arterial tube increases with its
distance from the heart, the velocity, from this cause, as has
already been observed, is continually diminished; but a contrary
effect takes place in the veins; for the different branches uniting,
for