Part 7

The softness of the mucous corion is also very variable; in the nasal fossæ, in the stomach and the intestines, it is really a kind of organized velvet. The name of villous membrane is perfectly suited to it. On the contrary, at the origins of the mucous system, as in the mouth, upon the glans penis, at the entrance of the nose, it is a more dense and compact texture, approximating nearer in its nature to the cutaneous corion. I am persuaded that like this it might be tanned and be useful in the arts if it were in larger pieces, whilst I doubt if the action of tanning could produce an analogous phenomenon upon the mucous texture of the deep-seated organs. The softness of this would render it incapable of serving for external integuments. The least cause would be in fact sufficient to break and tear it. Its structure being different from that of the cutaneous corion is the reason that the variolus pustules never appear on it, whilst we often see them on the mucous surfaces near the openings of the skin, especially upon the tongue, the palate and the internal surface of the cheeks.

Exposed to the action of dry air, so that it may come everywhere in contact with it, it becomes dry and very thin, but preserves some resistance. In bladders inflated and dried, in the stomach, the intestines, &c. thus prepared, it is this texture which supports these organs, and which prevents them from becoming flat, though we permit the air to escape; it offers even a resistance, from which arises a kind of crepitation when we wish to bend them in various directions. To be convinced of this, it is only necessary to dry the mucous surface separate from the serous and muscular that correspond with it; these when dried remain pliable like the cellular texture, whilst the first has a kind of rigidity.

In the organs in which the redness of the mucous texture is slight, as in the bladder, the rectum, &c. it becomes transparent by drying. Where it is very red as in the stomach, it takes a deep tinge, which becomes even almost black if there had been an antecedent inflammation in it by which much blood had been accumulated; hence it appears that this fluid is the cause of the colour.

Thus dried the mucous surfaces are smooth; they lose their viscidity at least in appearance; their folds are effaced by adhering to the surface from which they arise; thus the valvulæ conniventes are marked in a dried intestine, only by a red line, without any apparent prominence. But if we macerate the intestines in this state, the folds form again and become visible.

Exposed to a moist air, or left with other flesh that will not allow it to become dry, the mucous corion putrefies with great ease; the odour that it then sends out is very fetid. I think the reason why the abdomen of dead bodies becomes putrid so soon is in part because it contains substances already in putrefaction, and also because the surfaces, in contact with these substances, and which by their vitality resisted before their action, then readily yield to them. If these substances were contiguous to aponeuroses, putrefaction would be much less rapid. The mucous system when putrid takes a greyish colour; and as the dense, subjacent cellular texture putrefies much less easily, we can then remove from it by the least pressure, the mucous corion, reduced to a putrid pulp, in which every trace of organization has disappeared, and which forms a real pap.

During life, gangrene of the mucous texture takes place in general less frequently than that of the cutaneous. The consequences of catarrh, compared with those of erysipelas may convince us of this; there are however cases in which death appears in this texture, whilst the surrounding ones continue to live, as in gangrenous angina.

Exposed to maceration, the mucous texture yields to it promptly. I think that next to the brain it is altered quickest by the action of water. It is then reduced to a reddish pulp very different from that from putrefaction in the open air. When we put the whole stomach to macerate, this pulp is detached, when the sub-mucous texture and the serous membrane have as yet undergone but little alteration.

Ebullition at first extracts from the mucous texture a greenish scum, very different from that which the muscular and cellular textures give when boiled. This scum which mixed with the whole fluid in the beginning of the boiling, disturbs it and renders it green at first, afterwards rises upon the surface where it has small bubbles of air mixed with its substance; it often even falls to the bottom of the vessel by its weight. Sulphuric acid changes the colour of it to a dull brown.

A short time before the water begins to boil, the mucous texture crisps and acquires the horny hardness like the others, but in a less degree however; hence why it is then wrinkled almost always in different directions. In fact, the sub-mucous texture upon which it is applied, contracting at that time much more than it, it must fold on account of its length; thus during life, when the fleshy coat of the stomach contracts, its mucous surface not contracting in proportion, produces the numerous folds of which we have spoken. The action of a concentrated acid crisping the sub-mucous texture more than the mucous itself, produces an analogous phenomenon. After having been a long time dried, the mucous texture, like however almost all those of the animal economy, does not lose the faculty of acquiring the horny hardness when it is plunged into boiling water; it exhibits this phenomenon, whether we expose it to it dry, or whether we do it after having first softened it in cold water. It is a means by which all the valvulæ conniventes may be made suddenly to reappear, which had disappeared by drying, and which form again the instant the intestine contracts. This experiment is very curious to witness.

When the ebullition has been a long time continued, the mucous texture turns gradually to a very deep grey, from the white which it had first become. It is not softer than in the natural state, but it breaks much quicker; the following experiment is a proof of it. If we draw the mucous corion, boiled for some time with the subjacent cellular texture, this last resists the most; so that it remains entire, whilst the mucous corion is broken in many places. This never assumes the gelatinous appearance of the cutaneous corion or the fibrous and cartilaginous organs when boiled and of the others which yield much gelatine. However by mixing a solution of tannin with the water in which this system taken from an adult has been boiled, I have seen an evident precipitate.

The action of the acids reduces to a pulp the mucous texture much sooner than most of the others. During life, all the caustics act much more rapidly upon it than upon the cutaneous, of which the thick epidermis is an intermediate organ which checks their tendency of combining with its corion. Thus the instant the nitric acid, a substance which common people almost always choose for their poison, as the practice at the Hôtel Dieu proves, thus the instant I say, that the nitric acid is in contact with the alimentary canal, it disorganizes it, it forms there a whitish eschar, which, when death does not take place immediately, as most often happens, is slowly removed and detached in the form of a membrane. We know that the lips gently rubbed with weakened nitric acid, become the seat of a troublesome itching, whilst that oftentimes though this acid may have acted upon the skin sufficiently to make it yellow, it does not suffer from it.

The softness of the mucous corion makes me presume that it is easily altered by the digestive juices, not that I confide in the experiments of Hunter, who pretended that these juices could act upon the coat that secretes them, but because in general I have observed that the textures like it yield very easily to the action of water in maceration and are also very easily digested. I have not however any experiment upon the subject, and we know that in the animal economy analogy is not always a faithful guide.

All the mucous surfaces, but especially that of the stomach and intestines, have the property of curdling milk, as have many other substances, especially the acids. Is it to this property that must be attributed a phenomenon which is constant during life, viz. the coagulation of milk that enters the stomach for digestion? or is this phenomenon owing to the mixture of this fluid with those which are secreted by the surface of this organ? I think that both these causes contribute to it at the same time; both separate produce in fact this phenomenon. Spallanzani has proved it as it respects the gastric juices. Every body knows that the mucous membrane dried, and consequently deprived of these juices, preserves the property of coagulating milk. Spallanzani has convinced himself that the serous and organic muscular systems of the stomach are destitute of it.

Are aphthæ an affection of the mucous corion? do they belong to the papillæ? are they seated in the glands? are they a separate inflammation of these glands, whilst catarrhs are characterized by a general inflammation of a considerable extent of the mucous system? All these questions deserve to be examined. Pinel has perceived the void there is in morbid anatomy upon this point.

_Mucous Papillæ._

The peculiar kind of sensibility which the skin enjoys is attributed principally, as we know, to what is called the papillary body, which commonly is not easy to be demonstrated. The sensibility of the mucous membranes, somewhat analogous to that of the skin, appears to me to have the same kind of organization, which is perceived with infinitely less ease. The papillæ of this system cannot be called in question at its origin, where it dips into the cavities, in the commencement even of these cavities, as upon the tongue, the palate, the internal part of the alæ of the nose, upon the glans penis, in the fossa navicularis, within the lips, &c. Inspection is sufficient to demonstrate them there. But it is asked if the papillæ exist also in the deep-seated parts of this system. Analogy indicates it, since the sensibility is as great there as at their origin, though with varieties that we shall point out; but inspection proves it in a manner not less certain. I think that the villi with which we everywhere see them covered are nothing but these papillæ.

Very different ideas have been entertained of the nature of these villi; they have been considered in the stomach and the œsophagus as destined to the exhalation of the gastric juice, in the intestines as serving for the absorption of chyle, &c. But, 1st, it is difficult to conceive how an organ everywhere nearly similar, can perform in different parts functions so different; I say nearly similar, for we shall see that these papillæ exhibit differences of length, size, &c. without having any of texture or structure. 2d. What can be the functions of the villi of the pituitary membrane, of the internal coat of the urethra, the bladder, &c. if they have not relation to the sensibility of these membranes? 3d. The microscopical experiments of Leiberkuhn upon the vesicle of the intestinal villi have been contradicted by those of Hunter, of Cruikshank and especially of Hewson. I am certain that I have never seen any thing similar on the surface of the small intestines, at the time of chylous absorption; and yet it would appear that a thing of inspection could not vary. 4th. It is true that these intestinal villi are everywhere accompanied by a vascular net-work, which gives them a red colour very different from the colour of the cutaneous papillæ; but the non-appearance of the cutaneous net-work is owing to the pressure of the atmospheric air, and especially to the contraction which it occasions in the small vessels. Observe in fact the fœtus as it comes from the womb of its mother; its skin is as red as the mucous membranes, and if its papillæ were a little longer, it would resemble almost exactly the internal surface of the intestines. Who does not know moreover, that the vascular net-work surrounding the cutaneous papillæ, is rendered evident by fine injections, so as to change their colour entirely?

That in the stomach this vascular net-work continuous with the exhalants furnishes the gastric juice, and that in the intestines it intermixes with the origin of the absorbents, so that these embrace the villi, we cannot doubt after the experiments and observations of anatomists who have recently engaged themselves in investigating the lymphatic system. But this does not prevent the base of these villi from being nervous, and them from performing upon the mucous membranes the same functions that the papillæ do upon the cutaneous organ. This manner of regarding them by explaining their existence generally observed upon all the mucous surfaces, appears to me to be much more conformable to the plan of nature, than to suppose them in each place with different and often opposite functions.

Besides it is difficult to decide the question by ocular observation. The delicacy of these elongations conceals their structure, even from our microscopical instruments, agents from which anatomy and physiology do not appear to me to have derived much assistance, because when we see obscurely, each sees in his own way and according to his own wishes. It is then the observation of vital properties that should especially guide us; now, it is evident to judge by them, that the villi have the nature I have attributed to them. The following experiment served to demonstrate to me the influence of the papillary body upon the cutaneous sensibility; it succeeds also upon the mucous membranes. Remove the epidermis in any part and irritate the papillary body with a sharp stilet; the animal is agitated, cries out and gives marks of acute pain. Slide afterwards, through a small opening made in the skin, a pointed stilet into the sub-cutaneous cellular texture, and irritate the internal surface of the corion; the animal remains quiet and makes no noise, unless some nervous filaments accidentally struck make him suffer. Hence it follows very evidently, that it is upon the external surface of the skin that its sensibility resides, that the nerves pass through the corion without contributing to its texture, and that their expansion takes place on the papillary body. It is precisely so with the mucous surfaces. Observe that this circumstance coincides very well with the functions of the two surfaces, which receive by their free portion the action of external bodies, to which they are foreign by their adhering portion.

The papillæ exhibit very great varieties. On the tongue, in the small intestines, in the stomach and in the gall-bladder, they are remarkable for their length. The œsophagus, the large intestines, the bladder, all the excretory ducts have those that are less evident; these last especially and the urethra in particular, are almost smooth in their whole mucous surface. We can scarcely distinguish the papillæ in the frontal, sphenoidal, maxillary sinuses, &c.

These small nervous eminences are sufficiently distinct and separate upon the tongue. In the nasal fossæ, the stomach and the intestines, they are so near together and at the same time so delicate, that the membrane has at first view an uniform and smooth appearance, though it is covered with these elongations. Each papilla is simple; no bifurcation is ever observed at its extremity. All appear to have a pyramidal form, if we can judge at least by those which are the most evident.

Are they susceptible of a species of erection? It has been believed with regard to those of the tongue, which become erect, it is said, to perceive tastes, and with regard to those of the nose, which receive odours more efficaciously in this state of erection, which is in the sensitive phenomena on a small scale, what the erection of the corpora cavernosa is on a large one. I do not believe that any exact experiment can prove this fact. Moreover, it would be necessary then that the intestinal, vesical papillæ, &c. should be in permanent erection, since they are almost always in contact with foreign substances.

II. _Parts common to the Organization of the Mucous System._

Besides the blood vessels, the exhalants and the absorbents which contribute to the structure of this system as to that of all the others, it exhibits also a common organ, which is found almost always separate elsewhere, but which is here especially designed for it. This common organ is of a glandular nature; we shall now examine it.

_Of the Mucous Glands and of the Fluids which they secrete._

The mucous glands exist in all the system of this name. Situated beneath the corion or even in its thickness, they pour out incessantly through imperceptible openings a mucilaginous fluid which lubricates its free surface, and which defends it from the impression of the bodies with which it is in contact, and at the same time favours the course of these bodies.

These glands are very apparent in the bronchia, palate, the œsophagus and the intestines, in which they take the names of the anatomists who have accurately described them, and where they make in many places evident projections upon the mucous surface. They are less apparent in the bladder, the womb, the gall-bladder, the vesiculæ seminales, &c.; but the mucus that moistens them clearly demonstrates their existence. In fact, since on the one hand this fluid is analogous on all the mucous surfaces, and, on the other, in those in which the glands are apparent, it is evidently furnished by them, it must be secreted in the same way in those in which the glands are less evident. The identity of the secreted fluids supposes in fact the identity of the secretory organs. It appears that where these glands are hidden from our view, nature compensates for their delicacy by their number. Besides, there are animals in which, in the intestines especially, they form by their vast number, a kind of new layer, in addition to those of which we have spoken. In man this is remarkable in the palatine arch, in the pillars of the velum, on the internal surface of the lips, the cheeks, &c. &c. There is then this great difference between the mucous and the serous membranes, that the fluid which lubricates one is furnished by secretion, whilst that which moistens the others is from exhalation.

The size of the mucous glands varies; those of the velum of the palate, those of the mouth, &c. are the largest; they become insensible in the greatest number of mucous surfaces. I dissected two subjects that died of a pulmonary catarrh, and in them the glands of the trachea and bronchia, which are, as we know, very apparent, were not enlarged; the membrane alone appeared to be affected. Besides, we do not yet know the injuries of these glands, like those of the analogous organs, which are more apparent from their size. They are in general of a rounded form but with many varieties. No membrane appears to cover them. They have, like the salivary glands and the pancreas, only the cellular texture for an envelope. Their texture is more dense and compact than these last glands; but little cellular texture is found in them; they are soft, vascular, and appear when opened very much like the prostate gland. I cannot say whether nerves penetrate them; analogy indicates it, for all the principal glands receive them.

_Mucous Fluids._

We know but little of the composition of the mucous fluids, because in the natural state, it is difficult to collect them, and in the morbid, in which their quantity increases as in catarrhs for example, this composition is probably changed. We know that in general they are unsavoury, insipid, and but slightly soluble in water, in that even which is raised to the highest temperature; they become putrid with difficulty. In fact they remain a long time unchanged in the nose, exposed to the contact of a moist air; in the intestines, they serve, without danger to them, as an envelope for putrid substances, &c.; taken from the body and subjected to different experiments, they give results conformable to these facts. All the acids act upon them and colour them differently; exposed to a dry air, they thicken by evaporation, and are often even reduced to small shining laminæ. The nasal mucus especially exhibits this phenomenon. Fourcroy has given in detail the analysis of this mucus; he has also given that of the tracheal mucus. But we must not apply rigorously to the analogous fluids our knowledge of the composition of these. It is sufficient in fact to examine a certain number of these fluids, to be convinced that they are not the same in any two places, that more or less thick, more or less uniform, different in their colour, their odour even, &c. they vary in the principles that constitute them, as the membranes which furnish them vary in their structure, in the number and size of their glands, in the thickness of their corion, the form of their papillæ, the state of their vascular and nervous systems, &c. I am far from being certain that the gastric juice is a mucous juice; it is even probable that exhalation furnishes it, the glands of the stomach throwing out a different fluid by the way of secretion. But this assertion is not accurately demonstrated, and perhaps hereafter it will be proved that this juice, so different from the other mucous juices, is however one of them, and that its properties are distinct only because the structure of the mucous surface of the stomach is not the same as that of the other analogous surfaces.

The functions of the mucous fluids in the animal economy are not ambiguous. The first of these functions is to defend the mucous membranes from the impression of the bodies with which they are in contact, and all which, as we have observed, are heterogeneous to that of the animal. These fluids form upon their respective surfaces a layer which supplies, to a certain extent, by its extreme tenuity, the absence of their epidermis. Thus where this membrane is very apparent, as upon the lips, the glans penis, at the entrance of the nose and in general at all the origins of the mucous system, these fluids are not very abundant. The skin has only an oily layer, infinitely less evident than the mucous of which we are treating, because its epidermis is very distinct.

This use of the mucous fluids explains why they are more abundant where heterogeneous bodies remain some time, as in the bladder, at the extremity of the rectum, &c. than where these bodies are only to pass, as in the ureters, and the excretory ducts generally.