OF THE VITAL POWERS OF THE MUCOUS MEMBRANES.
57. The sensibility of mucous membranes is one of the principal
characteristics that distinguishes them from other analogous
organs. This power, which belongs to organic bodies, is variable
in every part, prompt to develop itself in some parts, under
the influence of the least excitement, roused with difficulty
in others, present in every part, liable to proceed by means
of inflammation from the most obscure state to the last degree
of intensity--this power is here remarkable for features very
analogous to those which it presents in the cutaneous surface (to
which, as we have stated, the mucous surface has great traits of
resemblance) as respects its structure. It is to this analogy of
sensibility that we must refer a crowd of phenomena, which are
alternately exhibited in an inverse order upon both surfaces. I
shall now point out some of these phenomena in succession.
58. (1) When the temperature of the surrounding air deadens the
sensibility of the cutaneous organ, by contracting its tissue, the
sensibility of the mucous surface receives a remarkable increase
of energy. Observe why in winter, and in cold climates, where the
functions of the skin are singularly limited, all those of the
mucous membranes are in proportion augmented; thence arises a more
evident pulmonary exhalation, the internal secretions are more
abundant, digestion is more active and more ready to operate,
consequently the appetite is the more easily excited. (2) When,
on the contrary, the heat of the climate, or of the season, &c.
relaxes and opens the cutaneous surface, we should say, that the
mucous surface is in proportion constricted: during summer, in
the south, &c. there is a diminution in the internal secretions,
the urine for instance; a tardiness in the digestive phenomena
by a default in the actions of the stomach and intestines, and
the appetite is slow in returning. (3) The sudden suppression of
the functions of the cutaneous organ often determines a morbid
increase of action in those of the mucous membrane. Cold air, which
checks the perspiration, frequently produces colds and catarrhs,
affections which are marked by the sensibility and increased action
of the mucous glands. (4) In various affections of the mucous
membranes, baths, which relax and determine to the skin, produce
beneficial effects.
59. The foregoing considerations evidently establish the influence,
which the vital powers of the skin have over those of the mucous
membranes. Others, not less important, demonstrate the reciprocal
dependence in which the skin is found with the same membranes, as
respects their vital powers. (1) During digestion, when the mucous
fluids are poured out in abundance into the stomach and intestines,
when, consequently, the mucous membranes of the alimentary canal
are in high action, the fluid of insensible perspiration is
evidently diminished, according to the observation of Santorius:
it is very small in quantity three hours after a meal, so that
the action of the cutaneous organ is visibly less energetic. (2)
During sleep, when all the internal functions become more marked
and are in full action, at which time the sensibility of the mucous
membranes is consequently highly excited, the skin appears to be
seized by a manifest debility--a debility, which is evinced by
the cold which it experiences when the animal reposes at night
uncovered, and by its want of susceptibility of various impressions.
60. The sensibility of the mucous membranes, like that of the
cutaneous organ, is essentially submissive to the immense influence
of habit, which, tending incessantly to blunt the acuteness of the
sensations of which they are the seat, reduces the pain and the
pleasure that we receive through them equally to indifference,
which is, as some say, the middle state.
61. I say, in the first place, that habit reduces the painful
sensations, which take place on mucous membranes, to indifference.
The presence of the catheter, which is passed up the urethra
for the first time, is cruel the first day, painful the second,
inconvenient the third, scarcely felt the fourth; pessaries
introduced into the vagina, bougies into the rectum, tents in the
nasal fossæ, the canula in the nasal canal, produce, in different
degrees, the same phenomena. It is upon this remark that is founded
the possibility of introducing instruments into the trachea to
aid respiration, and into the œsophagus to afford artificial
deglutition. This law of habit may even transform a painful into a
pleasant impression; of this fact the use of snuff, tobacco, and
various kinds of food, furnish us with remarkable examples.
62. In the second place I observe, that habit produces indifference
to those sensations on the mucous membranes which were at first
agreeable. The perfumer placed in a fragrant atmosphere, and the
cook, whose palate is constantly affected by delicious flavours, do
not experience, in their professions, the exquisite pleasures that
they prepare for others. Habit may even change pleasant sensations
to painful ones, as in the preceding paragraph we saw it changed
painful to pleasing sensations. I observe, further, that this
remarkable influence of habit is exercised only over sensations
produced by simple contact, and not over those produced by real
lesion of the mucous membranes: thus it does not ameliorate the
pain produced by stone in the bladder, nor that which attends
polypus in the uterus.
63. It is to this power of habit over the vital energies of
the mucous membranes that we must, in part, refer the gradual
diminution of their functions which accompanies advancing age. All
is susceptibility in the infant: in old age all is dull. In the one
the very active sensibility of the alimentary, biliary, urinary,
and salivary mucous surfaces, is that which principally produces
that rapidity with which the digestive and secretory phenomena
succeed each other. In the other this sensibility, weakened by the
habit of contact, does not so closely connect the same phenomena.
64. Does not the following remarkable modification of the
sensibility of the mucous surfaces depend upon the same cause,
_viz._ that at their origin, as on the pituitary membrane, the
glans, the anus, &c., they give us the sensations of bodies with
which they are in contact, and that they do not produce this
sensation in the deeply seated organs which they line, as the
intestines, &c.? In the interior of these organs this contact is
always uniform; the bladder is in contact with the urine only,
the gall bladder with the bile, the stomach with the aliments
masticated and reduced to an homogeneous, pulpy paste, whatever
may be their diversity. This uniformity of sensation prevents
perception, because, in order to perceive, we must compare, and
here two terms of comparison are wanting. Thus the fœtus has no
sensation of the liquor amnii: the air is also very irritating at
first to the new-born infant, but at length it is not felt. On the
contrary, at the origins of mucous membranes exciting agents vary
every instant: the mind can, therefore, perceive their presence,
because it is able to establish relations between their various
modes of action. What I say is so true, that if in the interior of
the organs the mucous membranes be in contact with a foreign body
differing from that which is habitual to them, they transmit the
sensation of it to the mind; instruments introduced into bladder or
stomach are examples of it. Fresh air, which in very hot weather is
suddenly introduced into the trachea, causes an agreeable sensation
over the surface of the bronchi; but from habit we soon become
insensible to it, and the perception ceases.
65. It is very difficult to point out with precision the character
of the tonic powers of mucous membranes, because, being almost in
every part united to a muscular layer, we can hardly distinguish
what belongs to the tonicity of the one from what depends upon the
irritability of the other; or otherwise, if the mucous membranes
be isolated, as in the nostrils, yet their attachment renders the
phenomena of their tonic powers very obscure. Nevertheless, the
action of the excretory ducts on their respective fluids, that
of the gall bladder, and of the vesiculæ seminales, which are
destitute of muscular attachments, and the spasmodic contraction
of the urethra, which sometimes takes place when the sound is
introduced, leave no doubt of the energy of this tonic power,
doubtless similar in its various modifications to that which is
observed in the cutaneous organ.