arteries, veins, and nerves: and from
this construction it is evident, that these membranes must possess great
irritability to peculiar stimuli, though they are incapable of any motions,
that are visible to the naked eye: and daily experience shews us, that in
their inflamed state they have the greatest sensibility to pain, as in the
pleurisy and paronychia.

3. On all these membranes a mucilaginous or aqueous fluid is secreted,
which moistens and lubricates their surfaces, as was explained in Section
XXIII. 2. Some have doubted, whether this mucus is separated from the blood
by an appropriated set of glands, or exudes through the membranes, or is an
abrasion or destruction of the surface of the membrane itself, which is
continually repaired on the other side of it, but the great analogy between
the capillary vessels, and the other glands, countenances the former
opinion; and evinces, that these capillaries are the glands, that secrete
it; to which we must add, that the blood in passing these capillary vessels
undergoes a change in its colour from florid to purple, and gives out a
quantity of heat; from whence, as in other glands, we must conclude that
something is secreted from it.

III. The seat of rheumatism is in the membranes, or upon them; but there
are three very distinct diseases, which commonly are confounded under this
name. First, when a membrane becomes affected with torpor, or inactivity of
the vessels which compose it, pain and coldness succeed, as in the
hemicrania, and other head-achs, which are generally termed nervous
rheumatism; they exist whether the part be at rest or in motion, and are
generally attended with other marks of debility.

Another rheumatism is said to exist, when inflammation and swelling, as
well as pain, affect some of the membranes of the joints, as of the ancles,
wrists, knees, elbows, and sometimes of the ribs. This is accompanied with
fever, is analogous to pleurisy and other inflammations, and is termed the
acute rheumatism.

A third disease is called chronic rheumatism, which is distinguished from
that first mentioned, as in this the pain only affects the patient during
the motion of the part, and from the second kind of rheumatism above
described, as it is not attended with quick pulse or inflammation. It is
generally believed to succeed the acute rheumatism of the same part, and
that some coagulable lymph, or cretaceous, or calculous material, has been
left on the membran