Part 6

The mucous membranes occupy the interior of the cavities which communicate with the skin by the different openings this covering has on the surface of the body. Their number at first view is very considerable; for the organs in which they are reflected are very numerous. The mouth, the stomach, the intestines, the œsophagus, the bladder, the urethra, the womb, the ureters, all the excretories, &c. &c. derive from these membranes a part of their structure. Yet if we consider that wherever they are continuous, wherever we see them originate, by extending from each other, as they originated in the first instance from the skin, we shall perceive that this number must be wonderfully limited. In fact by thus describing them, not separately in each part, but at the same time in all those upon which they are continued, we see that they are reduced to two general surfaces, of which all the others are portions, and which, on account of the different parts to which they are distributed, can be called, the one gastro-pulmonary, the other genito-urinary. The first is found in the head, the neck and the abdomen. This last cavity and more especially the pelvis, contain the second.

There is also a small insulated mucous surface; it is that which enters by the openings of the nipples and lines the lactiferous tubes. But it is so small that it deserves but little attention; besides, what will be said of the two others will be equally applicable to it. It is then useless to examine it in a general manner.

I. _Of the two General Mucous Membranes, the Gastro-Pulmonary and the Genito-Urinary._

The gastro-pulmonary surface penetrates into the interior by the mouth, the nose and the anterior face of the eye. 1st. It lines the first and second of these cavities, extends from the first to the excretory ducts of the parotid and sub-maxillary glands, from the other into all the sinuses, forms the conjunctiva, dips down into the puncta lachrymalia, the nasal duct, the sac of the same name, and is continued into the nose; 2d, it descends into the pharynx, furnishes an elongation to the Eustachian tube, then penetrates the internal ear and lines it, as we shall see; 3d, it dips down into the trachea and is spread upon all the air-tubes; 4th, it penetrates into the œsophagus and the stomach; 5th, it extends to the duodenum where it furnishes two elongations, one destined to the ductus choledochus, to the numerous branches of the hepatic and cystic ducts and the gall-bladder, the other to the pancreatic duct and its different branches; 6th, it is continued into the small and large intestines and finally terminates at the anus where we see it identified with the skin.

The second general mucous membrane, which we have called the genito-urinary, enters in man by the urethra, and thence is spread on the one hand upon the bladder, the ureters, the pelvis of the kidnies, the infundibula, the papillæ and the capillary tubes which open on their summit; on the other hand it goes into the excretory tubes of the prostate, into the seminal ducts, the vesiculæ seminales, the vasa deferentia and the branches with numberless windings that produce them.

In woman, this membrane is introduced by the os externum and penetrating on the one hand by the urethra, goes as in man upon the urinary organs; on the other hand, we see it enter the vagina, lining it, the uterus and the fallopian tubes, and afterwards becoming continuous with the peritoneum by the opening of these tubes. It is the only example in the economy, of a communication established between the mucous and serous surfaces.

This manner of describing the course of the mucous surfaces, by saying that they are elongated, dip down, penetrate, &c. from one cavity to another, is no doubt not conformable to the progress of nature, which creates in each organ the membranes belonging to this organ, and does not thus extend them by degrees; but our manner of understanding it is better adapted to this language, the meaning of which the least reflection will show.

In thus referring to two general membranes all the mucous surfaces, I am not only supported by anatomical inspection, but pathological observation has also furnished me with points of demarcation between the two, and points of contact between the different portions of membranes of which each is the assemblage. In the different descriptions of catarrhal epidemics given by authors, we frequently see one of these membranes affected in all parts, the other remaining sound; it is especially not rare to observe a general affection of the first, of that which extends from the mouth, the nose, the surface of the eye, into the alimentary canal and the bronchial vessels. The last epidemic observed at Paris, with which Pinal himself was affected, was of this character; that of 1761, described by Razou, had it also; that of 1752, described in the memoirs of the Society of Edinburgh was remarkable for a similar phenomenon; now, we do not see at that time a corresponding affection in the mucous membrane that is spread upon the urinary organs and upon those of generation. There is then here, 1st, an analogy between the portions of the first, by the uniformity of affection; 2d, a demarcation between the two by the health of the one and the disease of the other.

We see also that the irritation of any one point of one of these membranes, frequently produces a pain in another point of the same membrane, which is not irritated. Thus a calculus in the bladder occasions a pain at the end of the glans penis, the presence of worms in the intestines causes an itching of the nose, &c. &c. Now, in these phenomena purely sympathetic, it is very rare that the partial irritation of one of these two membranes affects with pain one of the parts of the other; there are however examples of it; such is the singular relation that exists, in mucous hemorrhages, between the membrane of the womb and that of the bronchia. If the blood ceases accidentally to flow from one during menstruation, the other frequently exhales it and thus as it were supplies its functions.

We ought then, from inspection and observation, to consider the mucous surface, in general, as formed by two great membranes successively spread upon many organs, having between them no communication except by the skin, which serves them as an intermediate organ, and which, continuing with both, contributes thus with them to form a general membrane everywhere, continuous, covering the animal externally, and extending within upon most of its essential parts. We can conceive that there should exist important relations between the internal and external portion of this single membrane; and that they do will be proved by further researches.

II. _Adhering Surface of the Mucous Membranes._

Every mucous membrane exhibits two surfaces, one adhering to the neighbouring organs, the other free, covered with villi, always moistened with a mucous fluid. Each deserves particular attention.

The adhering surface corresponds almost everywhere with the muscles, either of animal or organic life. The mouth, the pharynx, the whole alimentary canal, the bladder, the vagina, the womb, a portion of the urethra, &c. exhibit a muscular layer embracing on the outside their mucous tunic which is within. This arrangement coincides perfectly, in animals with a fleshy membrane, with that of the skin, which moreover approximates very near, as we shall see, the structure of the mucous membranes, and which, as we have seen, is everywhere continuous with them. This arrangement of the mucous membranes occasions them to be agitated by constant motions which favour wonderfully the secretion that takes place in them, the excretion which succeeds it, and the various other functions of which they are the seat. The insertion of this muscular layer exterior to the mucous system, is made, as we have seen, by this dense and compact texture which I have called the sub-mucous. It is from this texture, more compact than the rest of the cellular system, that the mucous surface derives its force. It is from it that the organ which it lines receives its form; it is this which supports and preserves this form; the following experiment proves it. Take a portion of the intestine; remove from any part of it this layer, as well as the serous and the muscular; then inflate it, after having tied it below; the air produces in this place a hernia of the mucous coat. Try afterwards another portion of intestine; deprive this, for a small space, of its mucous membrane and of this also; inflation will produce upon the serous and muscular tunics the same phenomenon that it did in the preceding on the mucous; then it is to this sub-mucous cellular layer that it owes the resistance with which it opposes substances that it contains. The same may be said of the stomach, the bladder, the œsophagus, &c.

III. _Free Surface of the Mucous Membranes._

The free surface of the mucous membranes, that which is continually moistened by the fluid from which they borrow their name, exhibits three species of wrinkles or folds.

1st. One inherent in the structure of all the laminæ of these membranes, is constantly met with, whatever may be their state of dilatation or contraction; such are those of the pylorus and the valve of Bauhin. These folds are formed not only by the mucous membrane, but also by the intermediate tunic of which we have spoken, which here has a remarkable density and thickness, and gives them solidity. The fleshy tunic enters even into their composition, and we see on the exterior, upon the serous surface, a depression that indicates their presence.

2d. Other folds, only formed by the mucous surface, exist also always in a state of vacuity or fulness, less evident however than this; they are owing to the circumstance that the mucous surface is much more extensive than those upon which it is applied, so that it is folded that it may not run a longer course; such are the valvulæ conniventes of the small intestines, the structure of which we see very well by cutting longitudinally one of these intestines. The edge of the section exhibits the fleshy layer and the serous surface in a straight direction, whilst the mucous layer describes a line resembling a loose thread.

3d. The last species of folds is as it were accidental, and is only observed during the contraction of the organ which is lined by the mucous surface that is the seat of it; such are those of the interior of the stomach, the great intestines, &c. In the greatest number of subjects brought to the dissecting rooms, these folds of which so much has been said as it respects the stomach, cannot be perceived in it, because the subject has died after a disease that has so altered the vital forces, as to prevent all action of this viscus; so that though it is frequently found empty, its fibres are not contracted. In experiments upon living animals, on the contrary, these folds become very evident, and may be demonstrated in this way; make a dog eat or drink copiously, open him an instant after and cut the stomach in the length of its great curvature; no fold is then apparent; but soon the viscus contracts, its edges are turned over and the aliments escape; the whole mucous surface is covered with an infinite number of very prominent ridges, which have as it were the form of cerebral circumvolutions. We obtain the same result by taking out the stomach of an animal recently killed, distending it with air and opening it afterwards, or by cutting it immediately in its state of vacuity and drawing it in opposite directions by its edges; it stretches, its ridges disappear, and if we cease to distend it, they form again immediately in an evident manner. I would observe on the subject of the inflation of the stomach, that by distending it with oxygen, we do not produce by the contact of this gas, greater ridges and consequently a stronger contraction, than by using for the same purpose carbonic acid gas. This experiment furnishes a result very similar to what I have observed in rendering animals emphysematous by different aeriform fluids. It follows from all that we have said upon the folds of the mucous membranes, that in the ordinary contraction of the hollow organs which these membranes line they undergo but a very slight diminution of surface, that they contract scarcely at all, but fold within, so that by dissecting them on an organ in contraction we should have a surface almost equal in extent to that which they exhibit during its dilatation. This assertion, which is true as it respects the stomach, the œsophagus and the great intestines, is not perhaps as entirely so with regard to the bladder, the contraction of which makes the ridges within less evident; but they are not sufficient to destroy the general law. It is also nearly the same with the gall-bladder; yet here we find another cause. The gall-bladder, alternately observed in hunger and during digestion, contains double the bile in the first case that it does in the second, as I have had occasion to see very often indeed, in experiments made for this object or with other views. Now, when the bladder is in part empty, it does not contract upon the bile that remains, with the force of the stomach when it contains but few aliments, or with the power of the bladder when it contains but little urine. It is then flaccid; so that its being distended or not has but a slight influence upon the folds of its mucous membrane.

Besides, in saying that the mucous membranes always exhibit, with some slight difference, the same surface in the extension and contraction of their respective organs, I intend to speak only of the ordinary state of the functions, and not of those enormous dilatations of which we often see the stomach, the bladder, and more rarely the intestines become the seat. Then there is no doubt, a real extension and contraction, which coincide in the membrane with those of the organ.

A remarkable observation which the free surface of the mucous membranes exhibits, and which I have already pointed out, is, that this surface is everywhere in contact with bodies heterogenous to that of the animal, whether these bodies introduced from without to nourish it have not yet been assimilated to its substance, as we see in the alimentary canal and in the wind-pipe; or whether they come from within, as we observe in all the excretory ducts of the glands, which all open into cavities lined by the mucous membranes, and transmit without particles which, after having contributed for some time to the composition of the solids, become heterogeneous to them, and are separated from them by the constant motion of decomposition which takes place in living bodies. Hence we ought to regard the mucous membranes as limits, and barriers, which, placed between our organs and the bodies that are foreign to them, defend them from the mischievous impression of these bodies, and serve consequently within, the same functions which the skin performs without, with respect to the bodies which surround that of the animal, and which tend incessantly to act upon it.

The organization of the mucous system and its vital properties are accommodated to this habitual contact of substances heterogeneous to the living economy. That which is a foreign body to the other systems, the cellular, the muscular, &c. is not so to this. Solid substances, the metals, stones, wood, &c. which introduced into our parts excite in them inevitably suppuration and an antecedent inflammation by their simple contact, pass with impunity over this, provided that their angles and asperities do not tear it; they only augment a little the secretion, as I shall say. We can swallow a ball of lead, of wood, &c. and pass it by the anus without inconvenience. All the irritating fluids without being caustic, that we inject into the great intestines in enemas, or that we swallow, would produce abscesses, &c. if they were forced into the cellular system, &c. Surgeons employ the term foreign bodies in too general a manner; that which is so for one system, is not for another. Foreign is, in this respect, a term of comparison which we should employ only after a knowledge of the peculiar sensibility of each system, and not after that of this property described in a vague manner.

The mucous system not only bears without danger the presence of all bodies that are introduced into the economy, but also when it goes out, it can be exposed with impunity to the contact of external stimuli. Observe what takes place in prolapsus of the womb in which the whole membrane of the vagina sometimes becomes external, in those inversions of the intestinal tubes that take place through preternatural ani, in prolapsus of the rectum, &c.; then the mucous surfaces serve really as integuments; now in these cases the surrounding bodies produce hardly any more pain on them than upon the skin. On the contrary, the instant a serous surface is laid bare, as for example in the operation of hernia, in which the intestine is suffered to come out, on account of an opening unfortunately made by the point of a bistoury, this surface inevitably inflames. All the cellular, muscular, nervous, glandular systems, &c. laid bare, exhibit the same phenomenon. There is no danger in opening the bladder as it respects the contact of the air, whilst there is much in permitting this fluid to penetrate into an articular cavity, a tendinous groove, a serous sac, &c. We know how much in the high operation for the stone, we fear to wound the peritoneum, how uncertain are the results of empyema on account of the contact of the air upon the pleura, &c. The dangers of the action of this fluid upon these surfaces have been perhaps exaggerated, but they are notwithstanding real.

If a fistula extends from the exterior of the abdomen into the intestines, its whole course is lined with callous bodies; these defend the cellular texture and the muscles through which the fistula passes. On the contrary, nothing similar appears upon the intestinal mucous surface, because its organization alone is sufficient to protect it. The urinary, salivary and lacrymal fluids never escape externally by artificial ducts formed in the neighbouring organs, without there are similar callous bodies in the course of these ducts; on the contrary, they pass with impunity over the mucous surfaces. Make in a limb a long and straight opening with a pointed instrument, and fix in it for a time a sound; a callous canal will be produced by it. Let a sound on the contrary remain in the urethra, no alteration of texture will result from it.

Let us conclude from these different considerations, that the mucous system with the cutaneous alone, is organized so as to support the contact of all external bodies, and not to be affected by their presence, or at least experience only an increase of secretion, which is in no ways dangerous. Thus these two systems form two boundaries, the one internal and the other external, between which are placed the organs foreign, by their mode of sensibility and by that of their structure, to the external bodies. To these boundaries the excitement of these bodies is limited; their influence does not go beyond. So long as they do not pass these boundaries, the other organs do not feel them. We might say that the acute sensibility which each of them enjoys, is a kind of sentinel that nature has placed on the confines of the organic domain of the mind, to inform it of whatever can injure it.

ARTICLE SECOND.

ORGANIZATION OF THE MUCOUS SYSTEM.

I. _Texture Peculiar to this Organization._

The mucous system presents two things to be considered in its peculiar texture, viz. 1st, a layer more or less thick which constitutes principally this texture, and which by analogy with the cutaneous corion, may be called the mucous corion; 2d, many small elongations which surmount it, and which are called villi or papillæ. As to the epidermis which covers it, I shall treat of it with the cutaneous epidermis. This texture has nothing similar to the substance that colours the skin, and which is between the papillæ and epidermis. We know in fact that negroes, as well as whites, have this texture of a bright red, which it derives from its vessels.

_Mucous Corion._

This portion of the mucous texture, which is the most important, and which constitutes the thickness, form, and even the nature of it, has a soft and spongy appearance. We might say at first view that it was a consistent pulp, with which the extremely dense cellular texture that is subjacent to it had been covered. This softness is a character which distinguishes it from the cutaneous corion, which moreover has by its intimate nature but little resemblance to it.

The mucous corion is very various as to thickness; it differs in this respect in each organ. That of the gums and the palate is the thickest of all. Then come those of the nasal fossæ and the stomach, then those of the small intestines and the gall-bladder, then those of the large intestines, of the urinary bladder, the urethra and the other excretories, until it begins at length to become so fine as to appear transparent like a serous surface when it is removed with care. Finally, the finest and most delicate is that of the sinuses of the face and the interior of the ear; the arachnoides is often coarser.

I have called the texture within the ear mucous, though all anatomists call the membrane of the cavities of this organ periosteum. In fact, 1st, we see it evidently continued with the pituitary membrane, by means of the elongation of the Eustachian tube. 2d. We find it constantly moistened by a mucous fluid which this canal serves to convey out, a character foreign to the periosteum, which, like the fibrous membranes, always adheres by both its surfaces. 3d. No fibre can be distinguished in it. 4th. Its fungous appearance, though white and soft, and the ease with which it is torn, are evident attributes of the mucous membranes. Every thing proves that the membrane of the tympanum, that of the meatus, &c. belong to the system of which we are treating. Thus in catarrhs of the pituitary membrane, and of that of the fauces, we most often find that the ear is affected; thus the ear is, like the mucous surfaces, the seat of hemorrhage, thus polypi arise in it, as in the nose and on the surface of the womb. We consider as a sign of deposition of pus in the ear, every purulent discharge coming from that cavity. But how can we conceive of a purulent collection in a part in which there is hardly any cellular texture, in a part wholly osseous? Besides the fibrous system, to which the periosteum of the tympanum belongs, hardly ever suppurates, as we know. Every thing then induces us to believe that these discharges are only the effect of a catarrh of the ear, a catarrh which is sometimes acute and sometimes chronic. I have moreover a recent and decisive fact upon this point; the body of a man subject to these discharges during life exhibited a remarkable thickness and redness of the membrane of the tympanum, but without the least trace of erosion. The ear suppurates like the urethra, the vagina, &c. it is not a new fluid which is formed by suppuration; it is that which naturally comes from the meatus which increases in quantity and comes through a preternatural opening of the membrane of the tympanum.

Diseases make the thickness of all the mucous surfaces vary remarkably. I have seen them many lines thick in the maxillary sinus, and nearly half an inch in the bladder. In great extensions of the mucous sacs, this thickness diminishes very much; it increases in their contractions. The stomach especially exhibits this phenomenon in these two opposite states.