OF THE EXTERIOR ORGANIZATION OF MUCOUS MEMBRANES.
8. Every mucous membrane presents two surfaces; the one adhering
to the adjacent organs; the other free, beset with villosities,
and always moist with a mucous fluid: each of them deserves a
particular attention.
9. The adherent surface is attached to muscles almost throughout
its extent. The mouth, the pharynx, the whole of the alimentary
canal, the bladder, the vagina, the uterus, and part of the
urethra, &c. present a muscular bed, embracing the exterior of
their mucous coat. In animals that have the panniculus carnosus,
this disposition perfectly coincides with that of the skin, which,
as we shall see, is in other respects analogous in structure
to mucous membranes. In man the cutaneous organ presents here
and there traces of this exterior muscle, as we observe in the
platysma myoides, the palmaris brevis, the occipito frontalis, in
most of the muscles of the face, &c. This disposition of mucous
membranes places them under the influence of those habitual changes
of contraction and dilatation, which are favourable to their
secretion, and various other functions.
10. This muscular bed is not immediately inserted into the exterior
surface of the mucous membranes, but rather, according to Albinus,
into a dense layer of cellular tissue, which all the ancient
authors have denominated, in the stomach, intestines, and bladder,
the nervous coat; but when well examined it presents no character
analogous to that which the name indicates. The experiment of
inflation, by which it is brought into its primitive state, is not
so easy as Albinus and others have pretended; which led me to think
that its nature might not be cellular, but that it was probably
of a fibrous texture, formed by a web of extremely delicate and
scarcely visible tendons, offering points of origin and insertion
to all the fleshy fibres of the muscular bed, which, as we know,
never describe entire circles, but rather different segments of
that curve. I confess that this conjecture, though very likely, is
not founded upon any decisive and rigorous experiment.
11. Whatever may be the nature of this intermediate membrane to
the mucous and muscular coats, it evidently has a dense, close
texture, which gives it a resistance very analogous to one of the
fibrous membranes. It is from this that the organ receives its
form; it is this which maintains and controls its shape, as may be
proved by the following experiment. Take a portion of intestine:
remove in any part of the bowel a part of this membrane, with the
serous and muscular membranes: having applied a ligature to the
inferior end, inflate it, the air will produce in the denuded part
an hernia of the mucous coat. Take another portion of intestine,
turn it, dissect off a small part of the mucous membrane and of
this coat: inflation will produce upon the serous and muscular
coats the same phenomenon as in the preceding case it did in the
mucous membrane. It is therefore to this intermediate tunic that
the mucous membrane owes its power of resistance to substances
which distend it. This applies equally to the stomach, bladder,
œsophagus, &c.
12. The free surface of mucous membranes, or that which is
continually moistened by the fluid from which they borrow their
name, presents two kinds of wrinkles or folds, the one inherent
in their structure and which is constantly present, whatever may
be their state of contraction or dilatation, such as the pylorus,
the valvula conniventes, the valve of the colon, &c. These folds
are formed, not merely by the mucous membranes, but also by the
intermediate membrane mentioned above, and which in these parts
takes a remarkable density and thickness.
13. The other folds may be called accidental, and are only observed
during the contraction of the organ; such are those of the inner
surface of the stomach, and of the large intestines, &c. In most
of the human subjects brought to our amphitheatres, these folds in
the stomach, of which so much has been said, are not perceptible,
because generally the subject has died of a disease which has
impaired the vital powers, without preventing all the action of
this viscus; so that, although it is frequently found empty, its
fibres are not in the least contracted.
14. In experiments on living animals, on the contrary, these folds
are very apparent; and observe how they may be demonstrated.
Let a dog eat or drink copiously; open it immediately, and make
an incision into the stomach the whole length of its greater
curvature, no fold will then appear, but it soon contracts, its
edges are drawn in, and the whole of the mucous surface is covered
with numerous prominent plicæ in the form of circumvolutions. The
same result may be observed in the stomach of a recently killed
animal by distending it with air, and then opening it; or, what is
still better, by laying it open whilst empty, and stretching it,
the folds will disappear, and when we cease to make the extension
they immediately form again and are very apparent.
15. I would observe on the subject of inflating the stomach, that
by distending it with oxygen gas the application of this fluid
does not produce more prominent folds, and therefore no stronger
contraction, than when carbonic acid gas is used for the same
purpose. This experiment presents a result very similar to what
I have observed when I have rendered animals emphysematous by
different æriform fluids. Frogs and Guinea pigs (these are the two
kinds I have chosen, the one being an animal of red and cold, and
the other of red and warm blood) presented very little difference
in their irritability, or their Galvanic susceptibility, whether
inflated with oxygen gas or with carbonic acid gas. They live very
well with this artificial emphysema, which gradually disappears.
Inflation with nitrous gas is always mortal, and its contact
appears to strike the muscles with atony. The stomach distended
with it very soon loses its power of contracting, and its folds
disappear. Here, as in all the experiments which have the vital
powers for their object, we frequently obtain very variable results.
16. It follows, from what we have said respecting the folds of
mucous membranes, that in the contraction of the hollow organs,
which are lined by them, they suffer but a very trifling diminution
of surface, they scarcely contract at all, but fold themselves
within; so that in dissecting them upon their contracted organ, we
have an extent of surface nearly equal to that which they present
during its dilatation. This assertion, which is true concerning
the stomach, the œsophagus, and the intestines, is, perhaps, not
quite so as respects the bladder, whose contraction does not show
within such prominent folds, but they are sufficiently marked to
bring the mucous membrane of this organ under the general law.
It is, also, nearly the same with the gall bladder; yet we find
here another cause; observed alternately, in a state of hunger and
during digestion, it will be found to contain double the quantity
of bile in the former case that it does in the latter, as I have
had the opportunity of seeing in numerous instances, in experiments
made with this object in view, or with other intentions. Now, when
it has evacuated part of its contents it does not contract upon
the remainder of the bile, with the energy of the stomach when it
contains but little food, nor with the power of the bladder when
it contains but a small quantity of urine, but is then flaccid,
so that its distention or nondistention has but very trifling
influence upon the folds of its mucous membranes.
17. Moreover, in saying that the mucous membranes present with
trifling variation the same extent of surface in the dilatations
as during the contraction of their respective organs, I intend to
speak of the ordinary state of the functions only, and not of those
enormous dilatations which are frequently seen in the stomach and
bladder, more rarely in the intestines. In such cases there is
doubtless a real extension, which in the membrane coincides with
that of the organ.
18. One remarkable observation that the free surface of mucous
membranes affords us, and which I have already pointed out,
is, that this face is everywhere in contact with bodies of a
different nature to that of the animal: these bodies are either
introduced from without for its nourishment, and are not yet
assimilated to its substance, as we see in the alimentary canal
and in the trachea, or they are produced within, as we observe in
the excretory ducts of the glands, which all open into cavities
lined by mucous membranes, and discharge those particles, which,
after having for some time formed a part of the composition of
the solids, become heterogeneous to them, and are thrown off
by that habitual action of decomposition, which takes place in
living bodies. According to this observation we must consider the
mucous membranes as defensive coats, placed between our organs and
foreign bodies, and that they consequently serve the same purpose
internally which the skin does externally, as respects bodies that
are in contact with it.