.

Now if it can be shown that inflammation, like vomiting and coughing,
is an effort of the system to remove an offending cause, and if we
can trace every step of this operation, with the exception of the
changes induced on the nervous system, we shall understand the nature
of inflammation as completely as that of any function of the body.

The circumstance the most difficult to explain, is the increased
redness of the part affected, which can only depend on an increased
quantity of blood in the vessels. This has been supposed to depend
upon an increased action of the vessels of the part; but that this is
not the case, must be evident from what was said when we were
speaking of the circulation of the blood. It was shown, that the
circulation could not be carried on by the mere force of elasticity
alone; this force, were it perfect, would produce no effect; but as
there is no body with which we are acquainted that is perfectly
elastic; so the coats of the arteries are very far from being so,
hence their effect as elastic tubes will be to diminish the force of
the heart, instead of adding to it; for a certain quantity of this
force will be spent in distending the vessels, which, were they
perfectly elastic, would be restored to them, but as this is not the
case, this force is by no means restored. Indeed a variety of
considerations, observations, and experiments, tend to prove, that
the vessels are endowed with a power very different from elasticity,
which differs only in degree from that of the heart; in short, they
are possessed of muscular power.

After each contraction of the muscular coat, the elastic will act as
its antagonist, and enlarge the diameter, till the vessel arrive at a
mean degree of dilatation, but after this there is no further power
of distention inherent in the vessel. The action of the elastic coat
ceases; and no one will assert that a muscular fibre has power to
distend itself.

The only power by which the vessel can be further distended, is the
vis a tergo: after the vessel arrives at its mean degree of
dilatation, both the elastic and muscular coats act as antagonists to
the vis a tergo, or force which propels the blood into, and thus
tends further to dilate the vessel. If then the vis a tergo become
greater than in health, the powers of resistance inherent in the
vessels remaining the same; or if the latter be weakened, the vis a
tergo, or propelling force, remaining the same, the vessel m