OF THE FUNCTIONS OF MUCOUS MEMBRANES.
70. I have already examined many of the functions of mucous
membranes. I have considered them (1) As one of the grand
emunctories of the animal economy. (2) As performing the same
functions with respect to heterogeneous bodies, which may be within
our organs, as the skin does with regard to the bodies with which
it may be in contact. (3) As facilitating the passage of foreign
bodies by means of the mucous fluid by which they are lubricated.
It remains for me to examine three questions much agitated at
this time. (1) If the mucous membranes have any influence over the
redness of the blood. (2) If they exhale. (3) If the absorbents
arise from them; and if absorption consequently takes place there.
71. The remarkable redness of these membranes, the analogy of
respiration, during which the blood becomes changed in colour
through the mucous surface of the bronchi, the well-known
experiment of a bladder filled with blood and placed in oxygen gas,
by which this fluid becomes also changed in colour,--have led to
the belief, that the blood, being separated from the atmospheric
air merely by a very fine pellicle on certain mucous surfaces,
as the pituitary membrane, the palatine, the glans, &c., would
there also take a brighter red colour, either by parting with a
portion of carbonic acid gas, or by combining with the oxygen of
the atmosphere, and that these membranes thus fulfilled functions
accessory to those of the lungs. The experiments of Jurine upon the
cutaneous organ, experiments adopted by many celebrated physicians,
appear also to favour the reality of that conjecture.
72. Observe the experiment that I have tried, in order to ascertain
the validity of that fact. Through a wound in the abdomen I drew
out a portion of intestine, which I tied at one point. I then
returned it, keeping back a part, which I punctured, and introduced
into it sufficient atmospheric air to distend all that portion of
the bowel between the ligature and the orifice. I then confined the
air by another ligature, and reduced the whole. At the end of an
hour the animal was opened. I compared the blood of the mesenteric
veins, which arise from that portion of intestine distended by
air, with the blood of the other mesenteric veins arising from
the remainder of the canal: no difference of colour could be
observed: the internal surface of the inflated intestine did not
exhibit a brighter red. I expected to obtain a more marked effect
by repeating the same experiment on another animal with oxygen
gas, but I did not perceive any variation in the colour of the
blood. As on the mucous membranes, which are ordinarily in contact
with the air, this fluid is constantly renewed, and is agitated
by a perpetual movement, I tried to produce the same effect in
the intestines; for which purpose I made two openings into the
abdomen, through each of which I drew a portion of the intestinal
tube. I opened these two portions, adapting to one the tube of a
bladder filled with oxygen gas, and to the other that of an empty
bladder. I then pressed the full bladder so as to make the oxygen
gas pass into the empty one through the intermediate portion of
intestine which was in the abdomen, so that the warmth there might
encourage the circulation. The oxygen gas was in this manner sent
many times from one bladder to the other, making a current through
the intestine, which from its contraction was more difficult than
it at first appeared to be. The abdomen was then opened, but no
difference was found between the venous blood returning from that
portion of the intestine, and that which flowed from the other
parts of the canal. The superficial situation of the mesenteric
veins, which are covered by only a fine transparent lamina of
peritoneum, and their volume when the animal is not fat, render
these comparisons very easy to be made.
73. I think, that from what occurs in the intestines we cannot
infer what takes place in the pituitary and palatine membranes,
&c.; because, although analogous, their organization may be
different. In these parts we cannot examine the venous blood
returning from them, as in the intestines: but, (1) If we consider,
that in animals, which have for some time respired oxygen gas, the
mucous membrane of the fauces does not exhibit any increase of
redness; (2) If we bear in mind, that the lividity of different
parts of this membrane, in those asphyxias which are produced by
carbonic acid gas, is not occasioned by the immediate contact of
this gas with the membranes, but by the reflux towards the surface,
of the venous blood which cannot pass through the heart, as occurs
in submersion, as demonstrated by Godwin, and as takes place in
all those cases in which the blood, previous to death, has found
difficulty in passing through the lungs; (3) If we remark lastly,
that in these circumstances the contact of the air, after death,
does not alter the lividity that the venous blood gives to the
mucous membranes, although the skin is then more permeable to
every kind of æriform fluid;--we shall see that we must at least
suspend our judgment, respecting the colouring of the blood through
mucous membranes, until farther observations shall have decided the
question.
74. Observe another experiment, which may throw more light still
upon the subject. I have distended the peritoneal cavity of
different Guinea pigs with carbonic acid gas, with hydrogen gas,
with oxygen gas, and with atmospheric air, to see if I could
obtain, through a serous membrane, what I had not been able to
effect through a mucous surface. In these experiments I have found
no difference in the colour of the blood of the abdominal system:
it was the same as in fresh animals of the same kind, that I
always used to compare with those on which the experiments were
made.
75. I believe, nevertheless, that I have observed many times, both
in frogs and in animals with warm and red blood, such as cats and
Guinea pigs, that the infiltration of oxygen gas into the cellular
tissue gives, after a certain time, a brighter colour to the blood
than this fluid presents in the artificial emphysemas which may
be produced by carbonic acid gas, hydrogen gas, or by atmospheric
air, in which circumstances the blood differs very little in colour
from its natural shade. But in other cases oxygen gas has had no
influence over the colour of the blood; so that, notwithstanding
the many experiments that have been made on this point, I cannot
state any general result. It appears, that the tonic powers of
the cellular tissue, and of the coats of the vessels which ramify
in it, receive a very varied influence from the contact of the
gases, and that, according to the nature of that influence, the
fibres contracting and becoming more or less firm render these
parts more or less permeable, both to the æriform fluids, which
have a tendency to escape from the blood to unite with that of the
emphysema, and to this last fluid, if it tends to combine with
the blood. This will doubtless explain the variations that I have
observed.
76. Do the mucous surfaces exhale? The analogy of the skin would
seem to lead to the belief of it; for it appears well proved, that
the perspiration is not a transudation by the inorganic pores of
the cutaneous surface, but a true transmission by vessels of a
particular nature, and continuous with the arterial system.
77. It appears, at first, that the pulmonary perspiration which
takes place on the surface of the bronchi, which has such
connection with that of the skin, which increases or diminishes
according to the decrease or augmentation of the other, and
of which the composition is apparently of the same nature--it
appears, I say, that the pulmonary perspiration is produced, at
least in part, by the system of exhalent vessels; and that if the
combination of the oxygen of the air concurs with the hydrogen of
the blood to produce it, during the act of respiration, it is but
in a very small quantity, and for that portion only which is purely
aqueous. It is necessary to observe further on this subject, that
the dissolution of the mucous fluid, which lubricates the bronchi,
in the air that is constantly inspired and expired, furnishes a
considerable portion of that vapour which rises from the lungs, and
which is insensible in summer, but very apparent in winter.
78. The intestinal juice, that Haller has particularly considered,
but which appears to be less in quantity than he had estimated,
the gastric juice, and that of the œsophagus, are very probably
disposed of by way of exhalation on their respective mucous
surfaces; but in general it is very difficult to distinguish with
precision, in these organs, what belongs to the exhalent system
from what is furnished by the system of mucous glands, which, as we
have said, are everywhere subjacent to them. Thus we constantly see
the mucous fluids of the œsophagus, stomach, and intestines, mix
themselves with the other fluids of these parts.
79. That mucous membranes absorb is evidently proved by the
absorption of the chyle upon the intestinal surfaces, of venereal
virus upon the glans and urethra, of variolous poison which is
sometimes rubbed upon the gums, of the serous portions of the
bile, of the urine, and of the semen, when they remain in their
respective reservoirs. When, from paralysis of the fleshy fibres
which terminate the rectum, the fæces accumulate at the extremity
of that intestine (a very common case in aged persons, and of which
Desault has cited many instances), these accumulations frequently
become hard, probably from the absorption of their juices, which
are obstructed there. We have many cases in which the urine has
been almost entirely absorbed by the mucous surface of the bladder,
when there has been absolute obstruction in the urethra. Whatever
may be the mode of this absorption, it appears that it is not
performed in a constant, uninterrupted manner, like that of the
serous membranes, in which the exhalent and absorbent systems are
in a continual alternate action; but that it occurs only under
certain circumstances, of which perhaps the greatest part are not
in the natural order of the functions. Finally, we have yet fewer
data respecting the mode of mucous absorption than on that of
cutaneous absorption: we confess it is very little understood, and
many even question its existence.