Part 2

27. We distinguish only a kind of gelatinous juice intermediate to the corps papillaire and epidermis, and most commonly it is not even apparent; I have never been able to observe more with certainty. In examining the skin of a Negro with attention, the epidermis being detached, I have seen the external surface of the chorion tinged with black, and that was all. Further, whatever this corpus mucosum may be, it certainly does not exist in mucous membranes, since they do not participate in the colour of the integuments. The heat of the sun, which darkens these in white people, does not appear to act upon the commencement of these membranes, which are equally exposed with them to its influence, as is seen in the red borders of the lips, &c. Nevertheless, I have many times remarked on the palates of dogs, which have been the subjects of my experiments, similar spots to those which have marked their skin.

28. The sensibility of the skin is principally owing to the corps papillaire; that of the mucous membranes, exactly analogous to that of the skin, appears to me to arise from the same cause. The nervous papillæ of these membranes cannot be questioned: at their origin, where they dip into the cavities, even in the commencement of these cavities, as on the tongue, the palate, the internal surface of the alæ nasi, on the glans, in the fossa naviculare, on the inside of the lips, &c., inspection is sufficient to demonstrate them. But, we ask, do these papillæ exist also in those parts of mucous membranes which are more remote from the surface of the body? Analogy answers in the affirmative, since sensibility is the same there as at their origin; but inspection proves it in a no less certain manner. I believe, that the villosities with which we see them everywhere thickly furnished are nothing else than these papillæ.

29. Very different notions have been entertained concerning the nature of these villosities: they have been considered, in the œsophagus and in the stomach, as destined to the exhalation of the gastric juice, in the intestines as serving for the absorption of chyle, &c. But (1) It is difficult to conceive how an organ, so nearly similar throughout its extent, should fulfil, in different parts, such different functions; I say so nearly similar, because we know, that the villosities of the small are more prominent than those of the large intestines. (2) What would be the functions of the villosities of the pituitary membrane, of the internal coat of the urethra, and of the bladder, if they had no connection with the sensibility of these membranes. (3) The microscopic experiments so boasted of by Leiberkuhn, on the erection of the intestinal villosities, have been contradicted by those of Hunter and Cruikshank, and, above all, by those of Hewson. I can assert, that I have never seen any thing of the kind on the surface of the small intestines during the absorption of chyle, and yet it appears to be a thing that cannot vary in different examinations. (4) It is true that these intestinal villosities are everywhere accompanied by a vascular web, which gives them a colour very different from that of the cutaneous papillæ; but the nonappearance of the cutaneous web is occasioned only by atmospherical pressure, by means of the contraction that it produces in the minute vessels: see, for instance, the newly-born infant; its cutaneous surface is as red as that of its mucous membranes, and if the papillæ were a little more elongated the skin would exactly resemble the internal surface of the intestines: moreover, who does not know, that the vascular web surrounding the papillæ is rendered so apparent by fine injections as entirely to change the colour of the skin?

30. That in the stomach this vascular web exhales the gastric juice, and in the intestines it is interlaced with the origin of the absorbents, so that they embrace the villosities, are facts that we must admit, after the experiments and observations of the anatomists, who in these times have been engaged with the lymphatic system: but that does not contradict the assertion, that the bases of these villosities are nervous, and perform the same functions only on the mucous membrane as the papillæ do on the cutaneous organ. This view of them, by explaining their existence as observed generally over all the mucous surfaces, appears to me much more conformable to the plan of nature than to suppose that they perform, in their different parts, diverse and frequently opposite functions.

31. However, it is difficult to decide the question by ocular observation; the tenuity of these prolongations conceals their structure even from our microscopic instruments, a kind of agents by which physiology and anatomy do not appear to me in other respects ever to have obtained great assistance, because when parts are so viewed each person sees in his own way, and is impressed accordingly. It is therefore the observing of the vital functions that should above all guide us. Now by judging of the villosities in this way it appears evident, that they have the nature which I have attributed to them. The following experiment will serve to demonstrate the influence of the corps papillaire upon the cutaneous sensibility: it succeeds also with mucous membranes. If we remove any part of the epidermis, and irritate the corps papillaire with a pointed instrument, the animal writhes, cries, and gives signs of acute pain. If afterwards the cutis be pierced, and with the instrument the internal surface of the chorion be irritated, the animal will not appear to suffer pain, unless by accident some nervous filaments should be touched. Thence it follows very evidently, that the sensibility of the skin resides in its external surface, that the nerves pass through the chorion without being interwoven with its texture, and that their diffusion only takes place on the corps papillaire. It is the same in mucous surfaces.

32. The length and form of the villosities vary in the different mucous surfaces. Their appearance is not the same in the stomach, the intestines, the bladder, the gall bladder, on the glans, &c.; which variation exactly coincides with the sensibility peculiar to each organ, a sensibility proved by numerous observations since Bordeu, who was the first to direct the attention of physiologists to the particular modifications that this property undergoes in the different parts.

33. Like the skin, the mucous membranes have their chorion: it is thick on the palate, gums, and pituitary membrane, delicate in the stomach and intestines, not very distinct in the bladder, gall bladder, and excretory ducts. It appears to be formed of condensed cellular strata, strongly united, as in the skin. Maceration develops this texture in a very sensible manner. There is nevertheless this difference, that in dropsy the cutaneous chorion rises and resolves itself into distinct cellules, that become filled with water, whilst no such change takes place in the mucous chorion under similar circumstances. Does this difference in the morbid state suppose a dissimilarity of structure? Certainly not; for the synovial membrane is evidently of the same nature as the serous membranes; and nevertheless it does not participate in the hydropic diathesis which often affects them universally. It would be curious to expose mucous membranes to the action of tan, to see if they would present the same phenomena as the skin.

SECTION IV.

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.

SECTION V.

OF THE VASCULAR SYSTEM OF MUCOUS MEMBRANES.

45. The mucous membranes receive a great number of vessels: the remarkable redness which distinguishes them would be sufficient to prove it to us, if it could not be demonstrated by injections. This redness is not everywhere uniform; it is less in the bladder, large intestines, and frontal sinuses; very marked in the stomach, small intestines, and vagina, &c. It is produced by a web of very numerous vessels, whose supplying branches, after having passed through the chorion, finish on its surface by an infinite division, embracing the corps papillaire, and is covered only by the epidermis.

46. It is the superficial position of these vessels that frequently exposes them to hæmorrhages, as we remark principally in the nose, and as is seen in hæmoptysis, hæmatemæsis, hæmaturia, in certain dysenteries, where the blood escapes from the parieties of the intestines, in uterine hæmorrhages, &c.; so that those spontaneous hæmorrhages, which are independent of any external violence applied to the open vessels, appear to be special affections of the mucous membranes; they are seldom observed but in these organs, and they form at least one of the grand characteristics which distinguishes them from all the other membranes.

47. It is also the superficial situation of the vascular system of mucous membranes that renders their visible portions, as on the lips, the glans, &c.; serviceable in showing us the state of the circulation. Thus, in various kinds of asphyxia, in submersion, strangulation, &c., these parts present a remarkable lividity; the effect of the difficulty that the venous blood finds in passing through the lungs, and of its reflux towards the surfaces where the venous system arises from that of the arteries.

48. I have already observed in the fœtus, and newly born infant, that the vascular system is as apparent in the cutaneous organ as in the mucous membranes; that the redness is there the same; it is even in that part more marked in the earlier periods of conception; but soon after birth all the redness of the skin seems to concentrate itself upon the mucous membranes, which before, being inactive, had no need of so considerable a circulation, but which, becoming all at once the principal seat of the phenomena of digestion, of the excretion of the bile, of the urine, of the saliva, &c., demand a larger quantity of blood. The long continued exposure of mucous membranes to the air frequently occasions them to lose their characteristic redness, and they then assume the colour of the skin (as M. Sabattier has well observed in treating on prolapses of the uterus and vagina). By this circumstance some have been deceived in believing such instances to be cases of Hermaphrodism.

49. An important question in the history of the vascular system of the mucous membranes presents itself, which is, does this system admit more or less blood, according to its various circumstances? As the organs within which this sort of membrane is spread are nearly all of them susceptible of contraction and dilatation, as is observable in the stomach, intestines, bladder, &c., it has been believed, that during their dilatation the vessels, being more spread out, received more blood, and that during their contraction, on the contrary, being folded on themselves, and as it were strangulated, they admitted but a small portion of this fluid, which then flows back into the adjacent organs. M. Chaussier has applied these principles to the stomach, the circulation of which he has considered as being alternately the inverse of that of the omentum, which receives, during the vacuity of that organ, the blood which it, being in a state of contraction, cannot admit. Since M. Lieutaud, an analogous use has been attributed to the spleen. Observe what I have ascertained on this subject from the inspection of animals opened during abstinence, and in the various periods of digestion.