in consequence of stimulus, like all other muscular
fibres; but, as they have no antagonist muscles, the cavities of the
vessels, which they form, would remain for ever closed, after they have
contracted themselves, unless some extraneous power be applied to again
distend them. This extraneous power in respect to the heart is the current
of blood, which is perpetually absorbed by the veins from the various
glands and capillaries, and pushed into the heart by a power probably very
similar to that, which raises the sap in vegetables in the spring, which,
according to Dr. Hale's experiment on the stump of a vine, exerted a force
equal to a column of water above twenty feet high. This force of the
current of blood in the veins is partly produced by their absorbent power,
exerted at the beginning of every fine ramification; which may be conceived
to be a mouth absorbing blood, as the mouths of the lacteals and lymphatics
absorb chyle and lymph. And partly by their intermitted compression by the
pulsations of their generally concomitant arteries; by which the blood is
perpetually propelled towards the heart, as the valves in many veins, and
the absorbent mouths in them all, will not suffer it to return.

The blood, thus forcibly injected into the chambers of the heart, distends
this combination of hollow muscles; till by the stimulus of distention they
contract themselves; and, pushing forwards the blood into the arteries,
exert sufficient force to overcome in less than a second of time the vis
inertiae, and perhaps some elasticity, of the very extensive ramifications
of the two great systems of the aortal and pulmonary arteries. The power
necessary to do this in so short a time must be considerable, and has been
variously estimated by different physiologists.

The muscular coats of the arterial system are then brought into action by
the stimulus of distention, and propel the blood to the mouths, or through
the convolutions, which precede the secretory apertures of the various
glands and capillaries.

In the vessels of the liver there is no intervention of the heart; but the
vena portarum, which does the office of an artery, is distended by the
blood poured into it from the mesenteric veins, and is by this distention
stimulated to contract itself, and propel the blood to the mouths of the
numerous glands, which compose that viscus.

II. The glandular system of vessels may be divided into those, which take
some fluid from t