OF THE GLANDS OF MUCOUS MEMBRANES.
34. Besides the three strata, which we have just mentioned, the
mucous membranes present in their structure a great number of
glands and blood vessels. The mucous glands exist in all membranes
which bear that appellation: they are situate under their chorion,
and even in its substance: they continually discharge, through
imperceptible orifices, a mucilaginous fluid, which lubricates
their free surface, and defends it from the impression of the
bodies with which it is in contact, at the same time that it
facilitates the passage of those substances.
35. These glands, which are very apparent in the bronchi, palate,
œsophagus, and intestines, where they take the name of the
anatomists who have particularly described them, are less obvious
in the bladder, the gall bladder, uterus, vesiculæ seminales,
&c.; but the mucus which moistens the membranes unequivocally
demonstrates their existence. In fact, since this fluid is nearly
of the same nature on all the mucous surfaces, and, in those where
the glands are apparent, is evidently furnished by them, it must be
secreted in the same manner in those where they are less evident.
The identity of secreted fluids, certainly, supposes the identity
of the secreting organs. It should seem, that in situations
where these glands escape our observation, nature makes up for
their tenuity by increasing their number. In the lower animals,
particularly in the intestines, they form by their number a kind
of new layer, in addition to those we have described. The same may
be observed in the palate, velum, &c. in man.
36. There is therefore this great difference between mucous and
serous membranes; that the fluid which lubricates the former is
furnished by secretion, whilst that which moistens the latter is
produced by exhalation. We know but little of the composition
of mucous fluids, because in the natural state it is difficult
to collect them, and in the morbid state, where their quantity
increases, as for instance in catarrhs, their composition probably
undergoes some alteration: but their functions in the animal
economy are well ascertained.
37. The first of these functions is to defend the mucous membranes
from the impressions of the bodies with which they are in contact,
and which, as we have observed, are all heterogeneous to the
animal. Here, without doubt, we see the reason why the mucous
fluids are more abundant in the cavities where these bodies remain
for some time, as in the bladder, at the extremity of the rectum,
&c., than in those organs through which they merely pass, as in
the ureters, and in general in all the excretory ducts. Observe
again, why, when the impression of these bodies might be hurtful,
these fluids are poured out upon their surfaces in a much greater
quantity. The sound which is introduced into the urethra, and is
allowed to remain there; the instrument that is left in the vagina
to secure a polypus; that which, with a similar intention, remains
some time in the nasal fossæ; the canula, fixed in the lacrymal
sac, to remove the obstruction; and the tube that is introduced
into the œsophagus, when deglutition is interrupted, always
determine a more plentiful secretion upon the corresponding mucous
surface. This is one of the principal causes why it is so difficult
to retain elastic tubes in the trachea; the abundance of mucous
fluid, which is then separated, chokes up the apertures of the
instrument, and renders its frequent removal necessary, and may
even threaten the patient with suffocation, as Desault has himself
observed, although he has nevertheless many times succeeded with
that operation.
38. It therefore appears, that every acute excitement of mucous
surfaces determines, in the corresponding glands, a remarkable
augmentation of action. But how can this excitement, which does
not take place immediately upon the glands, have so great an
influence over them? For, as we have said, these glands are always
subjacent to the membrane, and are consequently separated by it
from the irritating bodies. It appears that the above fact belongs
to a general modification of the glandular sensibility, which is
susceptible of being put into action by every irritation upon the
extremities of the excretory ducts, which will be proved by the
following considerations: (1) The presence of food in the mouth
produces a more abundant flow of saliva. (2) The catheter fixed
in the bladder, and irritating the ureters, or their vicinity,
increases the flow of urine. (3) The introduction of a bougie, but
half way up the urethra, will often be sufficient to occasion the
bladder to contract with a power equal to force the urine through
the passage, and so to overcome an obstruction in the canal. (4)
The irritation of the glans, and of the extremity of the urethra,
sub coitu, determines the contraction of the vesiculæ seminales,
and augments the secretory action of the testes. (5) The action
of an irritating fluid on the tunica conjunctiva occasions an
abundant flow of tears. (6) In making experiments upon the state
of the abdominal viscera during digestion, and under the influence
of hunger, I have observed, that whilst the food is only in the
stomach there is very little flow of bile; but it increases
when the aliment passes into the duodenum, so that then there
is a considerable quantity in the intestines. During hunger the
gall bladder is distended, and but little bile flows into the
intestines. At the end of digestion, and even when that process
is half finished, the gall bladder contains but half of its full
quantity; yet it might be expected to empty itself more easily
during abstinence, for then the bile it contains is of a deep
green colour, very bitter, very acrid, and likely to irritate the
organ which encloses it. On the contrary, during, or immediately
after digestion, it is more clear, mild, and less irritating;
there must, therefore, be, during digestion, another stimulus: now
this stimulus is the aliment passing over the mouth of the ductus
communis choledochus[A].
39. Let us conclude, from these numerous considerations, that one
of the principal means that nature employs to augment the action
of the glands, and to excite that of their excretory ducts, is
irritation upon the extremities of these ducts. We must refer to
that cause the abundant secretion and excretion of mucous fluids
in the cases above stated. It is also to this susceptibility of
the mucous glands, to be excited by irritation at the extremities
of their excretory ducts, that we must attribute the artificial
catarrhs which are occasioned by the respiration of chlorine
gas; the flow of mucus which attends a polypus, any tumour in
the vagina, stone in the bladder, &c. The frequent occurrence of
leuchorrhea in women who use coition immoderately, the abundant
flow of mucus from the noses of those persons who take snuff, in
all these cases there is evidently an irritation of the mouths of
the mucous ducts.
40. The mucous membranes, by the continual secretion of which they
are the seat, perform a principal part in the animal economy.
They are to be regarded as one of the grand emunctories, by which
the residue of the nutriment constantly escapes from the body;
and consequently as one of the principal agents of that habitual
decomposition which carries away from living bodies the particles
which for some time formed part of the solids, but have at length
become heterogeneous to them.
41. Remark the fact, that none of the mucous fluids enter into
the circulation, but are thrown out externally; that of the
bladder, ureters, and urethra, with the urine; that of the vesiculæ
seminales and of the vassa defferentia with the semen; that of
the nostrils by the action of blowing the nose; that of the mouth
partly by evaporation, and partly by the anus with the excrements;
that of the bronchi by the pulmonary exhalation, which is effected
principally by the solution of this mucous fluid in the air of
respiration; those of the œsophagus, of the stomach, of the
intestines, of the gall bladder, &c., with the excrements of which
they frequently form, in the ordinary state, a part nearly equal to
the residue of the aliment; and they even compose almost the whole
of the matter voided in certain dysenteries and fevers, where the
quantity is evidently disproportionate to the food that has been
taken. Let us observe on this subject, that in the analysis of the
fluids, in contact with the membranes of which we speak, as the
urine, bile, gastric juice, &c., there are always some errors,
because it is very difficult, impossible even, to separate them
from the mucous fluids.
42. If we call to mind what has been said above, upon the extent
of the two general mucous surfaces, that they are equal and even
superior to the extent of the cutaneous organ; if we afterwards
contemplate these two grand surfaces, constantly throwing off the
mucous fluids, we shall see of what importance this evacuation
must be in the economy, and of what derangements its lesion may
become the source. It is doubtless to this law of nature, which
ordains that every mucous fluid shall be rejected externally, that
in the fœtus we must attribute the presence of the unctuous fluid,
of which the gall bladder is full, and of the meconium choking up
the intestines, &c., kinds of fluids which appear to be only a
collection of mucous juices, which, as they cannot be evacuated,
remain, until birth, upon the organs where they have been secreted.
43. It is not the mucous fluids only that are rejected externally;
almost all the fluids, separated from the mass of blood by the
means of secretion, have the same destiny: this is evident in
the most considerable part of the bile. It is very probable,
also, that the saliva, the pancreatic juice, and the tears, are
discharged with the fæces, and that it is their want of colour
alone that prevents them from being distinguished like the bile.
I do not know even if, in reflecting on a crowd of phenomena, one
would not be tempted to establish, as a general principle, that no
fluid, separated by secretion, returns into the circulation; that
this destination belongs only to fluids separated by exhalation,
as those of the serous cavities, of the articulations, of the
medullary organ, &c.; that all the fluids are thus excremental or
recremental, and that there is no recremental excrement, as the
common division points out[B].
44. What is certain, at least, is, (1) that I have never been able
to effect the absorption of bile or saliva by the lymphatics.
When I have injected them into the cellular tissue of an animal
they have always produced inflammation and suppuration. (2) We
know that the urine, when infiltrated, does not become absorbed,
and that it strikes with death every part that it touches; whilst
the infiltrations of lymph, or of blood, are readily absorbed.
(3) There is an essential difference between the blood and the
secreted fluids as concerns their decomposition, whilst exhaled
fluids and serum, &c., are in that respect very similar.