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18. One remarkable observation that the free surface of mucous membranes affords us, and which I have already pointed out, is, that this face is everywhere in contact with bodies of a different nature to that of the animal: these bodies are either introduced from without for its nourishment, and are not yet assimilated to its substance, as we see in the alimentary canal and in the trachea, or they are produced within, as we observe in the excretory ducts of the glands, which all open into cavities lined by mucous membranes, and discharge those particles, which, after having for some time formed a part of the composition of the solids, become heterogeneous to them, and are thrown off by that habitual action of decomposition, which takes place in living bodies. According to this observation we must consider the mucous membranes as defensive coats, placed between our organs and foreign bodies, and that they consequently serve the same purpose internally which the skin does externally, as respects bodies that are in contact with it.

SECTION III.
OF THE INTERIOR ORGANIZATION OF MUCOUS MEMBRANES

19. Between the mucous and other membranes, as respects their interior organization, there is this essential difference, that they are always formed by several thin fibrous layers; these layers or coats are, with the exception of the rete mucosum, the same as those which compose the skin with which these membranes have the most exact analogy. We are about to examine separately each of these layers, which are the epidermis, the corps papillaire, and the chorion, in their general attributes; we shall afterwards consider the particular modifications which they undergo in the different parts of the mucous surfaces.

20. All authors have admitted the epidermis of mucous membranes: it appears, even, that the greatest part of them have believed that it is merely that portion of the skin which descends into the cavities to line them; Haller in particular is of this opinion; but the least inspection is sufficient to show, that here, as in the skin, it forms but a layer superficial to the corps papillaire and chorion; boiling water, which detaches it from the surface of the palate, the tongue, and even from the pharynx, leaves the two other coats denuded and apparent.

21. This epidermis is very distinct upon the glans, at the anus, at the orifice of the urethra, at the entrances of the nasal fossæ, and of the mouth, and in general wherever the mucous membranes arise from the skin. It is demonstrated in these different places by the frequent excoriations which occur on them; it may be raised from the lips by a very fine lancet by the action of boiling water, a hot iron, or even by epispastics, as the method of the ancients proves, who employed them to produce a fresh raw surface for the cure of the hare lip.

22. But in proportion as we go into the depth of the mucous membranes, the existence of this coat becomes more difficult to be demonstrated; it cannot be raised by the finest instrument, nor detached by boiling water, at least in the gall bladder, in the stomach, and intestines. I have made these experiments in fresh slain animals, and also in those where the natural heat had quite left them. But what our experiments cannot effect, inflammations will often produce. All the authors, who have written on the affections of the organs which are lined by these membranes, mention instances in which flakes, more or less considerable, have been voided by the urethra, anus, mouth, nostrils, &c. Haller has collected a great number of similar observations. Without doubt the separation of the epidermis in these cases is produced nearly in the same way as we observe it in cutaneous inflammations. In many subjects that have died with symptoms of inflammation of the mucous membranes, and which I have already had the opportunity of dissecting, or of seeing dissected, I have not yet been able to observe this separation going on; that is to say, the epidermis separated at one point, and still remaining adherent at others, as in erysipelas. I have tried in vain to produce this effect by the application of an epispastic to the inner surface of the intestines of a dog.

23. This epidermis is subject, like that of the skin, to become callous by pressure. Choppart cites a case of a shepherd, "dont le canal de l'urètre présentoit cette disposition, à la suite de l'introduction fréquemment répétée d'une petite baguette pour se procurer des jouissances voluptueuses." We know the density that this envelope takes in the stomachs of the gallinacea. In certain circumstances, where the mucous membranes are protruded from the body, as in prolapsus ani, inversion of the vagina, in the artificial anus, &c., sometimes the pressure of the dress produces in this epidermis a thickness evidently more considerable than is natural to it.

24. The epidermis is attached to the hair on the skin, although it does not afford its immediate origin; sometimes also piliform productions are observed in the mucous membranes. The bladder, the stomach, the intestines, and the pituitary membrane have been in various instances the seat of these unnatural excrescences: Haller has cited various instances of them.

25. This envelope appears to have upon the mucous surfaces the same texture as on the skin, excepting in the delicacy of the laminæ from which it is produced. It is to this delicacy, which gives more exposure to the nerves, that we must doubtless refer the facility with which we excite various remarkable modifications in the sensibility, when by the Galvanic process we apply zinc to the mucous surface of the conjunctiva, the pituitary membrane, the internal membrane of the rectum, or of the gums, &c., and bring these several metal plates into mediate or immediate contact. The epidermis when removed is quickly reproduced; being destitute of all kinds of sensibility, it in this respect serves the same purpose as the skin, by guarding the very sensible corps papillaire which is subjacent to it. To its presence over the mucous membranes we must attribute the ability they have of being exposed to the air, and even to the contact of foreign bodies, without excoriating or inflaming, as is seen in cases of artificial anus, prolapsus ani, &c., whilst serous and fibrous membranes never suffer such exposure with impunity. Hence there is no danger, in this respect, from opening the bladder: hence, on the contrary, that precept so justly recommended, not to open the cavity of the peritoneum, and to make the least possible incision into the synovial capsules. I would observe, that the existence of the epidermis upon mucous membranes is an important consideration, as respects the opinion of those who, like Séguin, believing them to be without it, have said, that contagion is always received by the lungs, and not by the skin, which is, according to them, defended by this envelope.

26. In the organization of the skin, immediately under the epidermis is placed the corpus mucosum, particularly described by Malpighi, and generally considered as the seat of colour in the different varieties of the human species. It is described as a coat, pierced with holes by the passage of the nervous papillæ: M. Sabattier points out the manner of demonstrating it. Sömmering has, it is said, seen it separated from the epidermis and chorion on the scrotum of an Ethiopian. I confess that I have not yet been able to perceive it: M. Portal does not appear to have been more fortunate.

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.