OF THE VARIATIONS IN THE ORGANIZATION OF MUCOUS MEMBRANES IN
DIFFERENT REGIONS.
52. The assemblage of the epidermis, corps papillaire, chorion,
glands, and vessels, constitutes in the mucous membranes their
intimate organization, which presents very considerable variations
in the different regions in which they are examined. I shall point
out only the principal of them; for in no different parts do these
membranes present the same appearance, and in order to describe all
their differences they should all be examined.
53. One of these variations is that which the aspect of mucous
membranes presents at their origin, when compared with their
appearance in the more remote parts of the organs. Compare, for
instance, the surface of the glans, the inner surface of the
lips, the orifice of the urethra, &c., with any portion of the
inner surfaces of the stomach, intestines, &c. In the first the
corps papillaire will be seen slightly marked, and offering no
villous character, the epidermis thick, very distinct, and easily
separated, the chorion very evident, the vessels rather less
superficial, the mucous glands numerous and very large, more
especially in the mouth; in the other characters almost opposite
will be observed; we should say, that the mucous membranes have
at their origin a structure of a middle kind between the skin and
their deeper portions.
54. Another variation of structure, not less striking, is that
which is met with in that portion of mucous surface which lines
the sinuses. Here it has more redness, and an extreme tenuity;
the three layers cannot be distinguished; and although there is a
considerable secretion of mucous fluids, there are no perceptible
mucous glands. Such are the characters of those portions of the
pituitary membrane, which are considered as adapted to augment the
sensation of smell, but which do not perform that function in the
manner generally understood. In fact, the instant when an odour
enters the nose, having the air for its vehicle, it cannot at once
pass into the sinuses, because the orifices by which these cavities
communicate with the nose are very small; but it enters gradually,
impregnates all the air which they contain, and not being able to
escape readily, for the same reason that rendered its entrance
difficult, the sensation is prolonged, which on the general
pituitary membrane is soon dissipated by the action of the fresh
air. Thus therefore the pituitary membrane is destined to receive
the impressions of odours, and its extensions into the cavities of
the sinuses to retain them.
55. With regard to the particular structure of that portion of
mucous membrane which lines the sinuses I remark, that it is
absolutely the same as of that which is spread over the surface
of the internal ear, with the exception of a still more delicate
tissue. All anatomists call this membrane the periosteum of the
bony covering of the internal ear. The following considerations
prove that it is not a fibrous membrane, analogous to that which
covers the bones, but a mucous layer, like that of the sinuses. (1)
It is evidently seen to be a continuation of the pituitary membrane
by the medium of the Eustachian tube. (2) It is found to be
habitually moist with a mucous fluid, which is discharged through
that tube, a property foreign to fibrous membranes, both of whose
surfaces are always attached to some parts of the animal structure.
(3) No fibre can be distinguished in it. (4) Its spongy appearance,
though whitish, its softness, the readiness with which it gives way
to the least agent directed against it, with a view to tear it,
form a character not to be found in any part of the periosteum.
56. I pass over the other variations of structure in mucous
membranes in their different regions; in all they have real
differences. I observe only, (1) That these variations distinguish
them from serous membranes, whose aspect is everywhere the same, as
may be seen by comparing the pericardium with the peritoneum, &c.
(2) The sensibility of mucous membranes varies in a very peculiar
manner in their different portions: thus an emetic irritates the
stomach, but not the conjunctiva; the pituitary membrane perceives
only odours; the mucous surface of the tongue flavours, &c. On the
contrary, the contact of all kinds of bodies with the naked serous
membranes produces phenomena exactly analogous.