This system, the name of which I borrow, like that of the preceding, from the fluid that constantly lubricates one of its surfaces, is always like it arranged in the form of membranes, and never in fasciculi like the muscular system, or in round bodies like the glandular. It is formed by the peritoneum, the pleura, the pericardium, the arachnoides, the tunica vaginalis, &c. The term serous membrane will then be very often used to designate it. No one, I believe, before the publication of my Treatise on the Membranes, had considered in a general manner these organs, which perform a less important part in the functions than the mucous, but which in diseases are almost as frequently affected. Pinel, who has perceived the analogy of their inflammations, has taken this system as a character of one of the classes of his phlegmasiÆ.
ARTICLE FIRST.
OF THE EXTENT, FORMS, AND FLUIDS OF THE SEROUS SYSTEM.
The serous system occupies the exterior of most of the organs of which the mucous lines the interior; such are the stomach, the intestines, the bladder, the lungs, &c. We see it around all those that are essential to life, as around the brain, the heart, all the gastric viscera, the testicles, the bladder, &c. It does not form, like the mucous system, a surface everywhere continuous upon the numerous organs on which it is spread. But it is always found insulated in its different divisions which never have any communication. The number of these divisions is somewhat considerable. By considering in one view all the different serous surfaces, we see that as a whole they exceed the mucous surfaces viewed also in a general manner. One consideration is sufficient to convince us of it. The mucous and serous surfaces accompany each other in a very great number of parts, as in the stomach, the intestines, the lungs, the bladder, the gall-bladder, &c. so as to exhibit in them nearly the same extent. But on the one hand, the mucous surfaces extend where the serous are not met with, as in the nasal fossÆ, the oesophagus, the mouth, &c. &c.; and on the other, there is a very great number of serous surfaces existing separately from the mucous, as the pericardium, the arachnoides, &c. Now if we compare the extent of the separate serous surfaces, with that of the separate mucous surfaces, we shall see that the first is much greater than the other.
These considerations, apparently minute, deserve however particular attention, on account of the relation of functions existing between these two surfaces taken as a whole, a relation which is especially connected with the exhalation of the albuminous fluids produced by one, and with the secretion of the mucous fluids, of which the other is the seat. Besides, in examining the extent of each serous membrane in particular, we see great varieties from the peritoneum which has the greatest surface, to the tunica vaginalis which has the least.
The serous surface taken as a whole, compared with the cutaneous surface, is also evidently superior to it in extent; so that in this respect, the quantity of albuminous fluids constantly exhaled within, appears to be much more considerable than that of the fluid which is incessantly thrown off by insensible transpiration; I say in this respect, for different circumstances, by increasing the action of the cutaneous organ, can re-establish the equilibrium in the exhalation of these two fluids, one of which re-enters by absorption into the circulation, and the other is wholly excrementitious. I do not know even if the pulmonary and cutaneous exhalations united are not less than those which take place upon the serous surfaces.
Every serous membrane represents a sac without an opening, spread upon the respective organs that it embraces, and which are sometimes very numerous, as in the case of the peritoneum, sometimes single, as in the case of the pericardium, covering these organs so that they are not contained in its cavity, and so that if it was possible to dissect them from their surface, we should have this cavity whole. This sac has in this respect the same arrangement as those night caps, which are folded within themselves; a trifling comparison, but which gives an accurate idea of this sort of membranes.
From this general arrangement, it is easy to understand that the serous membranes are never opened to permit the vessels and nerves to penetrate the respective organs to which they go or from which they come off, but that they always wind round them and accompany them to the organ, and thus form for them a sheath which prevents them from being contained in their cavities; this removes the danger of infiltration of serum which lubricates them, an infiltration which would take place through the neighbouring cellular texture, especially if they were dropsical; if, as in the fibrous membranes, they were pierced with foramina for the passage of these vessels and nerves. This arrangement, exclusively remarkable in the membranes of which we are treating, and in the synovial ones, is evident at the entrance of the vessels of the lungs, the spleen, the intestines, the stomach, the testicles, &c. We see it very well in the arachnoides, a membrane essentially serous, as I have demonstrated elsewhere.
From the general idea that we have given of these membranes, it is also easy to understand how almost all are composed of two distinct parts, though continuous, and embracing, the one the internal surface of the cavity where they are found, the other the organs of this cavity; thus there is a costal and pulmonary pleura, a cranial and cerebral arachnoides, one portion of peritoneum spread upon the gastric organs, and another upon the abdominal parietes, a free portion of the pericardium, and one adhering to the heart. The same arrangement exists in the testicles, &c.
Though the serous membranes may be separate, yet there sometimes exists communications between them; that for example of the cavity of the omentum with that of the peritoneum, that of the cavity of the arachnoides with the cavity of the membrane which lines the ventricles by the canal that I have discovered, and the external orifice of which is seen below and at the posterior part of the corpus callosum; whilst the internal one is seen above the pineal gland, between the two rows of small round bodies which are usually found in this place.
There is but one example of continuity between the serous and mucous membranes, that which exists, by means of the Fallopian tube, between the peritoneum and the uterine surface. How does the respective nature of the two membranes change here?
Free Surface of the Serous Membranes.
Every serous membrane has one of its two surfaces free, everywhere contiguous to itself, and the other adhering to the neighbouring organs. The first is remarkable for its polish, which especially distinguishes this system and the following, from all the other membranes. All the organs which exhibit this arrangement owe it to the covering they borrow from it. The liver ceases to be smooth and shining at its diaphragmatic edge where the peritoneum abandons it. There is in this respect a great difference in the appearance of the anterior and posterior face of the cÆcum. The bladder is rough wherever the peritoneal covering is wanting. The cartilages of the ribs have not the polish of those of the articulations which the synovial membrane covers.
Does this remarkable attribute of the serous membranes depend on the compression exerted upon them? Their situation in places where they are exposed to continual friction, would seem to make it probable. Bordeu thought so, when he said that all the parts of the abdomen are originally covered with cellular texture, which by pressure is afterwards changed into membranes; so that the peritoneum is formed partially upon each gastric organ, and its different parts give birth, by uniting, to the general membrane. This explanation of the formation of the peritoneum is applicable, according to him, to the pleura, the pericardium, and all the analogous membranes. But if this is the progress of nature, 1st, why, whatever be the period at which we examine the foetus, do we find the peritoneum and the serous membranes as much developed in proportion, as their corresponding organs? 2d. How are the numerous folds of these membranes formed, such as the mesentery, the omentum, &c.? 3d. Why are there parts where they do not exist though they are exposed to as great friction as that of the parts where they are found? Why, for example, are the sides of the bladder destitute of it, whilst it covers its superior part? 4th. Why does it not also form serous surfaces around the great vessels of the arm, the thigh, &c. which impart to the neighbouring organs an evident motion? 5th. Why does not the thickness of the serous membranes increase where the motion is strongest and diminish where it is weakest? Why for example does the thickness of the tunica vaginalis equal that of the pericardium? 6th. How can friction internally produce an organized body, whilst externally it constantly disorganizes the epidermis? 7th. How can we associate the vascular lymphatic texture of the serous membranes with the pressure that produces them? The impossibility of resolving these numerous questions proves, that it is not to mechanical pressure that must be attributed the formation of the serous membranes and the polish of their surface; that their mode of origin is the same as that of the other organs; that they commence and are developed with them; that this polish is an evident result of their organization, as the mucous papillÆ depend upon the texture of the surfaces to which they belong. What would be said of a system in which these papillÆ should be attributed to the pressure of the aliments upon the stomach, of the urine on the bladder, the air on the pituitary membrane, &c.?
The free surface of the serous membranes separates entirely from the neighbouring organs those upon which these membranes are spread; so that they are to these organs real boundaries, barriers, if I may use the term, or integuments, if it should be preferred, very different however from those which are external. Observe in fact that all the principal viscera, the heart, the lungs, the brain, the gastric viscera, the testicles, &c. limited by their serous covering, suspended in the middle of the sac that it forms, only communicate with the adjacent parts where their vessels enter; everywhere else there is contiguity and not continuity.
This insulation of position coincides very well with the insulation of vitality which is remarked in all the organs, and especially in those that we have just noticed. Each has its peculiar life, which is the result of a particular modification of its vital forces, a modification which necessarily establishes one in the circulation, nutrition and temperature. No part feels, is moved and nourished like another, unless it belongs to the same system. Each organ executes on a small scale the phenomena which take place on a large one in the economy; each takes from the circulation the aliment that is proper for it, digests it, throws back into the mass of blood, the portion which is heterogeneous to it, and appropriates to itself that which can nourish it; it is digestion in miniature. No doubt the ancients wished to give an idea of this truth which has been so well explained by Bordeu, when they said that the womb was a living animal within another. A very important use then of the serous membranes is to contribute, by rendering independent the position of their respective organs, to the independence of the vital forces, life and functions of these organs.
Let us not forget to consider under the same point of view, the moist atmosphere with which they are constantly surrounded, an atmosphere analogous to that which the cellular texture forms for various other organs. In this atmosphere all the morbific emanations of the organ go and are lost, if we may so say, without these emanations injuring the other organs. We have seen that this atmosphere in the cellular system is sometimes the seat of phenomena wholly different, and serves to transmit diseases from one organ to another. Now the serous membranes are a barrier much less easily surmounted, because they have not filaments which go from one organ to another, there is only contiguity as I have said, with the organs that they surround. We very rarely see in the abdomen a disease of the liver communicated to the intestines, one of the spleen passing to the stomach, &c.
The smoothness of the free surface of the serous system greatly facilitates the motions of the organs which it covers. We have already observed that nature employs two principal means for this object, viz. the membranes and the cellular texture. By distributing externally the second of these means, it has designed the first especially for internal motions. The smoothness and moisture of the serous surfaces are singularly favourable for them. These internal motions are usually regarded only in an insulated manner, as relating to the functions of the organ that executes them, as in relation to the circulation for the heart, respiration for the lungs, digestion for the stomach, &c. But they should also be considered in a general manner; they should be regarded as carrying through the whole machine a continual excitement which supports and animates the forces and the action of all the organs of the head, the chest and the abdomen, which receive less sensibly than the organs of the extremities, the influence of external motions. It is these internal motions that excite, sustain, and develop within, the nutritive phenomena, as the motions of the thigh, the arm, &c. without, favour the nutrition of the muscles which are found there; this is seen very evidently in bakers and other mechanics who exert more particularly this or that part. It is thus that the serous membranes contribute indirectly to the nutrition and growth of their respective viscera; but they never have a direct influence upon this nutrition, because their organization and life are different from the life and organization of these viscera.
The free surface of the serous system differs essentially from that of the mucous, in this, that it contracts frequent adhesions. The pleura is of all the serous organs, that in which these adhesions are the most evident. We find almost as many dead bodies in which they exist, as we do those in which they do not. Next to the pleura is the peritoneum, then the pericardium, then the tunica vaginalis, then the arachnoides, which is that of all the serous surfaces in which adhesions are the least frequent, though I have observed them in it. These adhesions exhibit many varieties which can be studied best on the pleura, which are as follows.
1st. Sometimes the costal and pulmonary portion are so identified at many points or in every part, that they make but a single membrane, and are united as closely as the two edges of the lip in a hare-lip that has been operated upon with success. 2d. At other times the adhesion is so slight, that the least effort is sufficient to destroy it. I have many times noticed this fact in the pericardium. I saw it once in the tunica vaginalis of a man who had been operated upon for hydrocele by means of injection, at the time I was surgeon for operations at the HÔtel Dieu. Separated then from each other, the two surfaces were uneven; they lost their polish. 3d. Frequently between the costal and pulmonary portion of the pleura, between the surfaces of the peritoneum, &c. there are several elongations of various lengths, which form a kind of loose bridles, traversing the serous cavity, having the same organization and polish as the membrane of which they appear a kind of fold, containing in their interior a species of small canal, because they are formed by two layers united together, resembling very much the elongation of the synovial membrane of the knee, which goes from the posterior part of the patella to the space between the condyles of the femur, having also an appearance analogous to the different natural folds of the peritoneum. We can hardly conceive that these filaments so regularly organized can result from inflammation. I am inclined to believe that they are owing to an original conformation. 4th. Frequently between the two portions of the pleura, there are seen many other elongations wholly different, which are not smooth, and do not form canals, but which appear to be flocculent and really analogous to the cellular layers; so that where they exist it may be said, that the membrane is entirely changed into this texture, which is besides, as we shall see, the essential base of its organization. 5th. I do not speak of the adhesions produced by false membranes, by albuminous flakes, intermediate to the two portions of a serous surface, &c. These adhesions are to a certain point foreign to these surfaces.
II. Adherent Surface of the Serous System.
The external surface of the serous membranes adheres almost everywhere to the neighbouring organs; it is rare in fact to see these membranes detached on both sides. The arachnoides at the basis of the cranium, and some other examples are exceptions. This adhesion of the serous membranes to their respective organs, is wholly different from that of the fibrous membranes. In this last, the passage of the vessels so unites the two parts, that their organization, seems to be common, and when one is removed, the other almost always dies, as is seen in the periosteum in relation to the bones, &c. On the contrary, every serous membrane is almost foreign to the organ it surrounds; their organization is different. The following are proofs of it;
1st. We very often see these membranes abandon and cover again successively their respective organs; thus the broad ligaments, at a great distance from the womb in the ordinary state, are to it like a serous membrane during pregnancy. An intestine when distended borrows from the mesentery a covering that quits it when it contracts. The omentum is by turns, as Chaussier has well observed, a loose membrane in the abdomen and a covering of the stomach. The peritoneal envelope of the bladder often leaves it almost entirely. Has not the hernial sac of those enormous tumours of gastric viscera originally served to line the parietes of the abdomen? Now it is evident, since the different organs can exist separate from their serous membranes, that there is no connexion between their organization. 2d. It is always a loose texture, easily stretched in every direction, that serves as a means of union, and never a sanguineous vascular system, as in most of the other adhesions. 3d. The affection of an organ is not a necessary consequence of that of its serous membrane, and reciprocally the organ is often affected and the membrane does not become diseased. For example, in the operation for hydrocele, the testicle remains almost always sound in the midst of the inflammation of its tunica vaginalis. The inflammation of the mucous membrane of the intestines is not a consequence of that of their peritoneal covering; and reciprocally in the various acute catarrhal affections of the organs with a mucous membrane within and a serous one without, this last is never found inflamed. In a word, the affections of the mucous membranes are everywhere very distinct from those of the serous, though most commonly both contribute to the formation of the same organ. It is evident that a line of demarcation so great in the affections supposes one of course in the organization. The life of the serous membranes then is entirely distinct from that of their corresponding organs.
Yet there are cases where these membranes do not present this loose adhesion, and where they become so united to the organs which they line, that frequently the most delicate scalpel cannot separate them. Observe the tunica vaginalis on the albuginea, the arachnoides on the dura-mater, and other membranes which form what I have called the sero-fibrous, &c.; such is the connexion of these different surfaces, that many have been mistaken to the present time for a single membrane. There is however no more identity in the organization, than where the serous membranes are more loosely attached to their respective organs, as is seen in the peritoneum, the pleura, &c. Diseases sometimes make this difference very evident. I have seen the arachnoides in a subject that had been affected with a chronic inflammation, evidently thickened on the internal surface of the dura-mater, without this having experienced the slightest alteration; it was detached without difficulty and torn with great ease.
III. Serous Fluids.
Every serous membrane is moistened on its internal surface by a fluid almost the same as the serum of the blood. The exhaling orifices constantly pour it out and it is constantly taken up by the absorbents. Its quantity varies. A mere dew in the natural state, it is exhaled in vapour when the serous surfaces are laid bare and allow the air to dissolve it. It is in general more abundant in dead bodies than in the living, because on the one hand the transudation which the tonic forces prevent, then easily takes place from the destruction of these forces, and supplies the place of the vital exhalation, by transmitting mechanically by their weight, the fluids of the surrounding organs to the different serous cavities, and because on the other hand, this destruction of the tonic forces prevents every kind of absorption; hence the stagnation and accumulation of this fluid. We know to what an extent this increases in various dropsies, especially in that of the abdomen.
Does this quantity vary according to the different states of the organs which the serous membranes cover? It has been long said, that the synovia is exhaled in greater abundance in the motion of the articulations, than in their state of rest. I have no data on this point founded upon experiment; but I am certain that I have many times observed in living animals, that the exhalation of the serous surface of the abdomen does not increase during digestion, or at least if it is greater, absorption becomes more active, and thus the surface of the peritoneum is not more moist than at another time. I have opened the thorax of many small guinea-pigs, after having first made them run a long time in the chamber in order to accelerate their respiration, and I have not observed greater moisture on the pleura. Yet it cannot be doubted, as we shall see, that the quantity of the serous fluids may be very variable in the different acute diseases; that the serous membranes exhale more or less of them, according to the manner in which they are sympathetically affected.
In the first periods of inflammations, in which the exhalants of the serous membranes are full of blood which is preternaturally introduced into them, the serum does not ooze in greater quantity from their free surface. Then as they are on the one hand very sensible, and very dry on the other, the motions of the organs that they cover are wonderfully painful. It is in these first periods that adhesions take place. If they are not formed either on account of the motion, or for other reasons, and if resolution of the inflammation does not take place, then happens to the serous surfaces what happens to a wound not united; they suppurate, but this suppuration is never attended with ulceration or erosion of their substance. However abundant the purulent collections may be, these membranes always remain sound; their texture is only more or less thickened; pus is thrown out by them, like the natural serous fluids, that is by exhalation. We know how much this fluid varies in consistence from milky serum, to the thickest false membrane that adheres strongly to the surface that has exhaled the materials of it.
The nature of the fluids of the serous system is very evidently albuminous. The instant one of the membranes of this system is plunged into boiling water, I have observed that it is covered with a white layer which is concrete albumen, and which being removed some time after, leaves the surface nearly of its original colour. All the substances which coagulate albumen produce a similar layer upon the serous surfaces. The experiments of Hewson, who has collected some spoonsful of these fluids in the great animals, confirm their albuminous nature. Rouelle and Fourcroy who have analyzed the water of dropsies have also found albumen predominant in it. Observe upon this subject that all the white flakes swimming in this water, that the false membranes that form in it and the white fluids which give it the appearance of milk, appear to be only albumen which is found in different degrees of consistence. It might be said that the heat of inflammation has produced the same phenomenon during life, that common caloric does upon the white of an egg, the water of dropsies, &c. I shall not treat of the other accessory principles that enter into the composition of the serous fluids.
ARTICLE SECOND.
ORGANIZATION OF THE SEROUS SYSTEM.
The first characters of the structure of these membranes are a white, shining colour, less brilliant than that of the aponeuroses; a variable thickness, very evident upon the liver, the heart, the intestines, &c. hardly discoverable upon the arachnoides, the omentum, &c.; a remarkable transparency whenever these membranes are raised for a considerable extent, or are examined where they are detached on both sides, as on the omentum.
All have but a single layer which it is possible, at the places where it is thick, to raise from the cellular layers, but which can never be neatly divided into two or three portions; a character essentially distinct from those of the mucous membranes. The action of a blister on their external surface first laid bare, for example, on a portion of intestine drawn out in a living animal, does not make a pellicle rise upon it, as upon the skin, a pellicle under which the serum is collected. I have frequently made this attempt. What is the immediate structure of this single layer of the serous membranes? I shall now examine it.
I. Cellular Nature of the Serous Texture.
Every system is in general, as we have thus far seen, an assemblage, 1st. of common parts, which are especially the cellular texture, the blood vessels, the exhalants, the absorbents and the nerves, which form as we have said the outline and the frame of it, if I may so express myself; 2d. of a peculiar fibre formed by a substance which is deposited in this outline, by gelatine, for example, for the cartilages, by gelatine and phosphate of lime for the bones, by fibrin for the muscles, &c. That which makes these organs resemble each other then is the cellular organ, the vessels and the nerves; that which distinguishes them, is their peculiar texture, which depends itself upon a peculiar nutritive matter. A bone would become a muscle, if, without changing its texture at all, nature had imparted to it the faculty of secreting fibrin, and of encrusting itself with it, instead of separating the phosphate of lime and being penetrated with it. But the serous system does not appear to have in it a distinct nutritive matter, and consequently a peculiar texture. It is only formed of the mould, the outline of others, and is not penetrated by a substance that characterizes it. Almost wholly cellular, it does not differ from this system in its common form, except by a degree of condensation, by an approximation and union of cells which are found scattered in the ordinary state.
The following are the proofs that the texture of the serous system is wholly cellular. 1st. There is an identity of nature where there is an identity of functions and diseases; now it is evident that the uses of these membranes and of the cellular texture, as it respects the continual absorption and exhalation of lymph are completely the same, and that the phenomena of the various dropsies are common to them, with the difference only of the effusion in the one and infiltration in the other. 2d. The inflation of air into the texture subjacent to these membranes terminates by bringing them almost to a cellular state, when it succeeds and is pushed for some time; an experiment which is frequently very difficult. 3d. Maceration, as has been remarked by Haller, produces at length the same effect, but in a still more evident manner. 4th. The various cysts, hydatids, &c. whose appearance, texture and nature even are entirely the same as in the serous membranes, as we have seen, always arise in the midst of the cellular texture, grow at its expense and are wholly formed of it. 5th. No fibre is found in the serous membranes; a character that distinguishes it from all the other organs and analogous to that of the cellular texture.
To these various proofs of analogy, of identity even of the cellular and serous systems, we can add the action of different reagents, which give results precisely similar in both. 1st. Every serous membrane when dried, becomes transparent, does not turn yellow like the fibrous and the mucous membranes, preserves a pliability foreign to these membranes when dried, and gradually resumes its original state when it is immersed in water. 2d. It becomes putrid much slower than the mucous surfaces, the muscular layers, the glands, &c. This is remarkable in the abdomen, upon the peritoneum which is frequently almost untouched, when every thing is putrid around it, as may be seen by removing it; for its transparency would make you believe at first view that it was altered, if you examine it upon the fleshy and mucous surfaces. 3d. Maceration at the ordinary temperature of cellars, reduces with great difficulty to a pulp the serous membranes. The omentum, the finest and most delicate of these membranes has resisted it for a very long time in my experiments. This phenomenon is particularly striking when compared with the maceration of tendons which are so resisting, and which support such great efforts during life. These become pulpy in water before the omentum is touched. The same phenomenon takes place with regard to all the other serous surfaces. 4th. In boiling water, these surfaces acquire the horny hardness like the fibrous system, but furnish infinitely less gelatine; they do not become yellow like it. The pleura in those portions of the thorax of animals that are brought to our tables, has almost its ordinary appearance; only it is less shining, has lost the faculty of crisping from the action of caloric, is no longer affected in the same way by acids, &c. If it was of a fibrous nature it would have disappeared in gelatine, on account of its delicacy. I shall say the same of the external membrane of the spleen, the liver and the lungs. Compare these membranes, that are brought to our tables, when boiled with the intermuscular aponeuroses, the tendons, &c. you will see that it is impossible to confound, as has been done, all the white textures together, in regard to their nature.
If we compare the different effects of agents the most known upon the serous system, with those that we have observed upon the cellular system, we shall see that they are entirely the same; that these two systems are consequently analogous, and even identical.
The serous system when it putrefies in the open air does not become green like the skin, but is of a dull and very deep grey. During life, on the contrary, its blackness is very evident in gangrene which is sometimes the result of an acute inflammation, sometimes of those chronic inflammations, attended with many small white tubercles, which are so frequently found upon these membranes. This difference arises from the circumstance, that in the dead body these surfaces are not penetrated with blood at the time they become putrid; whereas they contain much during life, when putrefaction succeeds inflammation which has filled the exhalants with it. Many other facts prove, that the greater the quantity of blood there is in a part when it putrefies, the more livid and black it becomes. In the many dead bodies that I have opened, I have never yet observed gangrene except in the peritoneum. I have never seen it in the pleura, the arachnoides, the pericardium, the tunica vaginalis; it no doubt takes place in them; but I think I have opened dead bodies enough to allow my observation to establish as a general principle; that the peritoneum is more subject to it than all the other analogous organs.
Though the different considerations offered above establish much analogy between the cellular and the serous systems, they exhibit however real differences. First their external appearance is not the same. Then there is something in their intimate nature that we are unacquainted with, and which differs also; for whenever two organs are identical in their nature, they are subject to the same affections; now there is a disease of the serous surfaces that is not seen in the cellular system; it is those slow inflammations of which I spoke just now, a disease which should not be ranked in the class of the phlegmasiÆ, and which the production of the small tubercles that attend it, especially characterizes. Authors who have not sufficiently attended to it, have denominated it chronic enteritis in the peritoneum, latent inflammation in the pleura, &c. though however foreign to every subjacent organ, except in the latter periods when it is propagated by the cellular texture, it has its seat exclusively in the serous membranes, and is an affection peculiar to these membranes, as miliary eruptions are to the cutaneous surfaces, as aphthÆ to the mucous surfaces, &c. Add to this difference that of the pus which the cellular texture and the serous surfaces secrete; this fluid is not the same in the two systems. The difference of its nature is not known; but its external appearance is by no means the same.
II. Parts common to the Organization of the Serous System. Exhalants.
A very evident exhalation is constantly going on upon the serous surfaces. A particular order of vessels is the agent of this exhalation, the matter of which is the fluid mentioned above. These vessels are very distinctly demonstrated in this system; it is the only one in which the eye of the anatomist can accurately trace them. The following are the means of seeing them; 1st, in a living animal, draw out an intestine from the abdomen; it will have a reddish tinge owing to the vessels under the serous coat, and hardly at all to the vessels in this coat itself. Irritate it, and reduce the intestine after attaching a string to it, as in the operation of hernia where there is gangrene, draw it out again at the end of six and thirty or eight and forty hours; it will exhibit many reddish lines, running over this serous surface, and showing in it plainly the exhalants which were insensible in the natural state, on account of the transparency of their fluids. 2d. Very fine injections cover in an instant all the serous surfaces with an infinite number of lines of the colour of the injected fluid, lines which are evidently exhalants full of this fluid. 3d. In these injections an extremely fine dew is made to ooze out upon the smooth surface of the serous membranes, a dew which takes place without rupture or transudation, and of which the exhalants are the sources. 4th. If a serous surface is laid bare in a living animal, and wiped dry, it is soon after covered with new serum, which the exhalants furnish.
Absorbents.
From the texture of the serous membranes, it is evident that the lymphatic system enters essentially into their formation, and that they are probably only a net-work of exhalants and absorbents; for we have seen that the cellular organ is an assemblage of them. But this assertion which analogy dictates is also supported by direct proofs. 1st. The fluid of the dropsies of the different cavities varies in density and colour; now Mascagni has always observed that the lymphatics in their neighbourhood contained a fluid exactly analogous. 2d. The same author has found in two dead bodies, with a sanguineous effusion in the thorax, the absorbents of the lungs loaded with blood. 3d. In a man who had become emphysematous after having been poisoned, these vessels were distended with air. 4th. Coloured fluids injected into the abdomen or thorax are soon after found, it is said, in the neighbouring lymphatics, with the same colour. I have often repeated this experiment; the injected fluid has been soon absorbed, but not the matter which coloured it; so that this matter, more condensed after absorption, tinged the serous surface, the lymphatics being as transparent as usual. It is necessary in general to choose the abdomen for these experiments, because the absorbents being much exposed on the liver, can be more easily examined there. This absorbent faculty is preserved some time after death; but care should be taken, in order to obtain then the effect more certainly, to keep the animal, if a warm blooded one, in a bath of nearly its own temperature; I have had frequent opportunities of being convinced of this truth, and of observing with Cruickshank, that what Mascagni has said upon the absorption of dead human bodies, fifteen, thirty, forty-eight hours even after death, is at least very much exaggerated. 5th. The following experiment I make every year to demonstrate the absorbents; I macerate for five or six hours the heart of an ox in water; at the end of this time, the serous membrane of this organ, which hardly allowed these vessels to be perceived, appears to be covered with them. 6th. When the serous membranes are inflamed, the subjacent lymphatics are distended, like them, by the red globules of blood, &c. &c.
It appears then to be demonstrated, 1st, that the absorbents open by an infinite number of orifices upon the serous membranes; 2d, that their origins a thousand times intermixed with each other, and with the orifices of the exhalants, contribute especially to form their texture; 3d, that the difficulty of distinguishing the absorbent and exhalant pores is no reason for denying their existence, this difficulty arising from their extreme delicacy and from the oblique direction in which they open between the layers of these membranes; thus the obliquity of the insertion of the duct of Warton, and of the ductus choledochus even would render the inspection of them very difficult, though these ducts were infinitely larger; 4th, that from this structure, the serous membranes, always arranged as we have seen in the form of sacs without an opening, should be regarded as great reservoirs between the exhalant and absorbent systems, in which the lymph in going from one remains some time before entering the other, in which it undoubtedly undergoes various preparations of which we shall always be ignorant, because it would be necessary to analyze it comparatively in these two orders of vessels, which is almost impossible, at least for the first, and finally in which it serves different uses relative to the organs around which it forms a humid atmosphere.
Blood Vessels.
Do blood vessels enter into the structure of the serous membranes? These vessels are very numerous around them, as is seen in the peritoneum, the pericardium, the pleura, &c.; they wind upon their external surface and ramify there. But I have always doubted whether the greatest number of those which are thus contiguous to them, really make a part of their texture, and I am even convinced of the contrary. The following considerations support my opinion. 1st. When these vessels are injected, they can be easily raised with a scalpel from the external face of these membranes, without injuring their continuity, which can never be done in the fibrous or mucous membranes. 2d. No blood vessel is discoverable on these membranes which are free on both faces. The arachnoides at the base of the cranium furnishes an example of this. 3d. The vessels frequently change relations with these membranes. I have proved above that when the omentum is applied to the stomach when it is full, the vessels that are between its layers, do not mount with it upon this viscus, on account of the great stomachic coronary which opposes it. When dead bodies having large hernias are injected, the vessels that wind in the ordinary state upon the surface of the peritoneum which corresponds to the ring, are not seen extending below upon the hernial sac. Certainly the vessels that are observed in the broad ligaments of the womb, do not follow them in the great displacements they undergo in pregnancy.
I think it then very probable that the serous membranes have but very few blood vessels; what are called arteries of the peritoneum, the pleura, &c. are but trunks winding on their external surface, capable of abandoning it when they are displaced, being as it were foreign to them, not entering immediately into their structure, to which the absorbent, exhalant and cellular systems almost alone contribute. No doubt communications exist between the arterial system and the serous membranes, by means of the exhalants; but nothing precise is yet known upon the nature, arrangement, and to a certain extent even, the functions of these vessels.
III. Varieties of Organization of the Serous System.
We have seen the mucous system exhibiting in each part where it is found, numerous differences of structure and varying in each region and in each organ. The serous system varies also, though less than the preceding. 1st. Each membrane has its peculiar structure. Compare for example, the arachnoides and the peritoneum; the one fine, delicate and transparent, yields to the least effort, has no resistance, tears almost as soon as it is touched, never remains whole at the base of the cranium, where it is free, if the brain is raised ever so little, and has, when pressed between the fingers, a remarkable softness. The other, thicker and more compact, bears without breaking all the efforts imparted to the abdominal viscera; it can be pulled with impunity. Its texture is wholly different. 2d. The different portions of the serous membranes have not the same organization; the omentum is for example evidently dependant upon the peritoneum and yet it does not resemble it. I have observed that the intestinal portion of this membrane is much more delicate than its hepatic, mesenteric portions, &c. That the half of the tunica vaginalis which lines the albuginea and is identified with it, is certainly not the same as the half which is free on the side of the dartos muscle. I cannot say precisely in what these differences consist; but the external appearance is sufficient to establish them.
Ought we then to be surprised, if all the serous surfaces are not equally subject to the same diseases; if inflammation attacks them with such different degrees of violence; if it takes place ten times upon the pleura to once that it appears upon the arachnoides; if in the pericardium, the tunica vaginalis and the peritoneum, it does not exhibit the same symptoms; if dropsies vary also wonderfully in each; if the slow inflammations attack them differently, &c.? The pericardium is subject to an affection which I have seen upon no other serous surface, and which is yet extremely frequent upon this; I refer to the white layers, more or less broad, that are formed on its internal surface, which would be thought at first view to belong to its texture, which can however be raised from it leaving it sound. I do not know whence these layers come; do they correspond to the false membranes of the pleura?
Neither should we be surprised at what has been said of the varieties which the same membrane exhibits in its diseases. Frequently the whole of the peritoneum is diseased, and the omentum remains sound and vice versa. The layers of which I have just spoken are seen upon the cardiac portion, and not upon the free portion of the pericardium.
Observe however that all the diseases of this system have a common character which is evidently derived from the analogy of organization. This and the synovial are the only ones in which large serous collections take place, in which slow and tubercular inflammations are formed. The most of their modes of adhesion belong only to the serous system. Inflammation has a peculiar and distinctive character in it, of which all the serous membranes partake with some modifications. The inflammation of the meninges had been classed among the serous phlegmasias, from the analogy of the symptoms, before I had demonstrated that the arachnoides, one of these meninges, belongs essentially to the serous system. It is on account of this membrane, and not on account of the dura-mater which is of a fibrous nature, that phrenitis should be referred to the diaphanous membranes.
ARTICLE THIRD.
PROPERTIES OF THE SEROUS SYSTEM.
I. Properties of Texture. Extensibility.
The serous membranes are endowed with an extensibility much more limited, than the enormous dilatations of which they are capable in certain cases, would at first lead us to believe. The mechanism of their dilatation evidently proves it. This mechanism depends upon three principal causes; 1st, upon the development of the folds that they form, and this is the most powerful of the three causes. Hence why the peritoneum, which of all the membranes of this class, is the most exposed to dilatations, as from pregnancy, ascites and visceral enlargements, more frequent there than elsewhere; hence, I say, why the peritoneum exhibits so great a number of these folds, such as the mesentery, the mesocolon, the mesorectum, the two omentums, the fatty appendices, the fold of the cÆcal appendix, the broad ligaments of the womb, the posterior ones of the bladder, &c. &c. Hence why also these folds are seen around organs subject to habitual alternations of dilatation and contraction, as around the stomach, the intestines, the womb and the bladder; very evident in the second state, but slightly apparent in the first. 2d. The enlargement of the serous cavities belongs to the displacements of which their membranes are capable. Thus when the liver is considerably enlarged, its serous membrane increases its extent in part at the expense of that of the diaphragm, which being drawn is detached and applied upon the enlarged viscus. I have seen, in an aneurism of the heart, the pericardium which had been able to yield but very little, detached in part from the portion of the great vessels which it covered. 3d. Finally, the texture of these membranes undergoes a real distension and elongation. But it is in general the least sensible cause of the enlargement of their cavity; it is only in the great enlargements that it has an evident influence; in common cases, the two first causes are almost always sufficient.
I will make an important remark upon the subject of the displacements of which the serous membranes are the seat in the motions of their respective organs; it is that these displacements are very painful when these membranes are inflamed. When the dilated intestines separate the two diseased layers of the mesentery to lie between them, when the stomach goes between those of the omentum, &c. when the peritoneum is inflamed, the patient suffers much. Hence why flatulence is then so painful, why it is then necessary to avoid taking at once a great quantity of drink. We know the acute pain that a long inspiration produces in pleurisy; it is because the lungs then dilate the pleura, and tend to go between the folds which accompany the great pulmonary vessels.
Contractility.
It corresponds with the extensibility; it is less consequently than it at first appears to be. When the peritoneum for example is contracted, its different folds are formed; it returns to its place after having experienced locomotions. But it cannot be denied that in great dilatations these two properties are very sensible; for example, in hydrocele as the water is evacuated, the tunica vaginalis evidently contracts. The peritoneum after the paracentesis of the abdomen exhibits the same phenomenon. At the time of performing the operation of empyema, the pleura does not experience it so sensibly, not from defect of contractility, but because on the one hand it adheres to the ribs which do not contract, and on the other if the effusion is of long standing, the lungs are so flattened by the pressure, that the air can no longer dilate them, so that there remains a space between the costal and pulmonary portion, which is filled with air. A similar space would also remain at the moment of the operation, if the serum of hydrocephalus was evacuated.
After long distensions, the serous membranes no longer contract; the tunica vaginalis remains flaccid after frequent punctures, the peritoneum after frequent pregnancies, &c. &c.
II. Vital Properties.
The serous surfaces, being removed from the action of external bodies, do not enjoy in the natural state the properties which put the living organs in relation with external bodies; they have neither animal sensibility nor contractility. Thus they would be very improper for external integuments, or for linings of the organs which the mucous membranes cover; they would give in fact no other sensation than that of an obscure and indistinct feeling. They answer very well for envelopes, integuments for internal organs, but not for sensible envelopes. We have a proof of it in living animals in whom we can irritate these membranes with impunity. I have many times seen dogs in whom I had left the spleen drawn out of the abdomen, in order to observe the phenomena arising from it, tear this organ without being in a state of fury, eat it even and be thus nourished by their own substance. They also often tear without pain the exterior of their intestines when in experiments these are drawn out, and the animals are left some time to themselves.
When external bodies are in contact with the serous system, they change its natural state; they inflame it, as we see in the peritoneum, in the tunica vaginalis laid bare, as we observe also always when a foreign body introduced into the system acts upon them. Surgeons, as we know, even employ this means to which they would in vain have recourse in the mucous membranes, in order to procure artificial adhesions between the parietes of these membranes. The different morbid irritations inflame much more frequently the serous surfaces which in this state acquire a very acute sensibility, greater even than that of the integuments; so that these inflamed surfaces would be equally improper to serve for integuments, because external bodies would excite them painfully.
The sensible organic contractility is nothing in the serous system; but the insensible and the corresponding sensibility are kept in permanent exercise in it, 1st, by the continual exhalation and absorption that are going on there; 2d, by nutrition. These two properties are then those which predominate in this system; thus upon their alterations all its diseases turn. Acute inflammations, chronic inflammations with tubercles, adhesions, dropsies, exhalations of pus, of milky serum, &c. &c. are all derived from an excess, a defect or an alteration of these two properties of the serous system. It is then also almost exclusively that sympathies are put into action in it; so that the serous membranes diseased either idiopathically or sympathetically exhibit always a series of phenomena all of which suppose an increased internal motion, or loss of tone in the exhalant and absorbent capillaries, and in the peculiar texture of those membranes; whilst in the animal muscular, the organic muscular systems, &c. these predominant affections which are marked by convulsions and paralysis in one, and by irregular motions of irritability in the other, do not suppose this internal alteration of the texture of the diseased organ. Hence why these two last systems, though frequently disturbed during life, exhibit but few changes after death, whilst the serous system is a vast field for the morbid anatomist.
Sympathies.
The serous surfaces are capable of being influenced by the affections of the other organs; this is very evident in the organic diseases of the heart, the lungs, the liver, the spleen, the stomach, the womb, &c. organs, which without having any known connexion of functions with the serous surfaces, influence them however so that all their morbid defects of organization are accompanied, in the latter periods, by different serous collections in the great cavities, collections evidently owing to a derangement of the organs which constantly exhale this fluid. I shall make upon this point two observations; the first is that the serous surfaces nearest the diseased organ, are in general the most susceptible of being influenced by it. Thus in the diseases of the heart and the lungs, the serous collections take place especially in the thorax, whilst ascites is always the first consequence of enlargements of the liver, the spleen, &c. the pleura and the pericardium being filled subsequently. We know that most sarcoceles are complicated with dropsies of the tunica vaginalis; whence results hydro-sarcocele, a disease which surgeons consider separately, but which is the same as those of the preceding cases, which might in this respect be called hydro-phthisis, chronic hydro-hepatitis, hydro-carcinoma of the womb, &c.
The second observation that I have to make is, that whenever the serum is thus collected in the cavities, in consequence of an organic disease of a viscus foreign to the membrane, this serum is limpid, transparent, and probably of the same nature as that which circulates in the lymphatic vessels. The exhalants which form it not being then in fact diseased, their action not being increased or that of the absorbents diminished except by sympathy, the fluid must remain the same. Thus though there is suffering at the end of the glans from a stone in the bladder, the glans is perfectly sound, and the mucous fluid that escapes from it is of the same nature as in the ordinary state. On the contrary, when dropsies depend upon a disease of texture of the serous surfaces, as for example upon a tubercular inflammation, or even an acute one, which has degenerated, &c. the effused serum is almost always altered; it is milky, or there are albuminous flakes in it, or a false membrane, &c. I have made this observation, which I think interesting, upon nearly all the bodies I have opened.
In acute diseases, the serous surfaces receive also equally the sympathetic influence of the affected organs. If we could then see them, we should find that they were like the skin, more or less moist, more or less dry, according to the different periods of the disease. What proves it is, that at the death which follows the disease, the serum of the pleura, the pericardium, the peritoneum, &c. varies remarkably. Sometimes it is evidently increased, at others it is almost nothing; this depends upon the time in which the subject died. If it is whilst exhalation is very abundant, we find much serum; it is almost nothing if life has been sufficiently prolonged to allow absorption to take place. If the surrounding air did not dissolve the sweat, or if the skin was in the form of a sac, we should find it with very variable degrees of moisture, according as the subjects had died in sweat, or with a suppression of the cutaneous exhalation.
ARTICLE FOURTH.
DEVELOPMENT OF THE SEROUS SYSTEM.
I. State of this System in the First Age.
All the serous surfaces are extremely delicate in the foetus. In opening the thorax by a longitudinal section of the sternum and examining the pleura in the mediastinum where it is free on both sides, it is found to have less thickness than the transparent layers of the omentum or the arachnoides in the adult. The peritoneum is a little thicker in proportion, but yet its delicacy is very great. The comparison of soap bubbles is hardly sufficient to convey an idea of the fineness of the texture of the omentum and the arachnoides.
At this period the fluid that lubricates the serous surfaces is much more unctuous and viscid than it is afterwards; by carrying the fingers over these surfaces at the different ages, the difference is easily perceived. It might almost be said that the tangible qualities of the serous fluids then approximate those of the synovia. I know not to what this difference belongs.
Besides, the quantity of these fluids does not appear to be so great in proportion as that of the cellular fluids, with which they have however so much analogy; which is probably owing to this, that the internal motions being less numerous, on account of the inaction of most of the organic muscles, less fluid is necessary to lubricate the surfaces.
The growth of the serous system is always in proportion to that of the organs which it covers. The arachnoides is larger in proportion than it will be in the adult; it seems even, like the brain, to become then the seat of a more active labour; thus diseases are more frequent in it. The increase of exhalation is more common in it than in all the other serous sacs; hence hydrocephalus.
At birth, when the internal motions become suddenly very numerous, on account of respiration, digestion and the excretions, I presume that the serous surfaces become the seat of a more active exhalation. Besides, as very little blood penetrates them, the sudden production of the red blood and its entrance by the arterial system, where it succeeds the black blood, produces less changes upon them than upon the mucous surfaces and the muscular system.
The serous membranes grow like the other organs; for a long time delicate and diaphanous, they gradually thicken as we advance in age, and become of a dull white. Their suppleness diminishes as their density increases; they resist the different reagents so much the less as the subjects are younger. In infants, maceration and ebullition reduce them much more quickly to a homogeneous pulp.
I have observed that in the foetus which has become putrid, there is often collected different gases in the serous cavities, as may be proved by opening these cavities under water; a phenomenon much less evident in the adult, in whom the cellular texture is often wholly emphysematous by the putrefactive motion, without the escape of any thing by the canula of a trochar which is plunged into the peritoneal cavity or into that of the pleura, as I have many times ascertained. In general, there is disengaged much more aeriform fluid from the organs of the foetus, than from those of the adult, in the experiments of maceration.
II. State of the Serous System in the after Ages.
In the adult, the serous system remains a long time without undergoing any very sensible change; its membranes follow only the laws of the organs they surround. Thus in the age nearest youth, the serous surfaces of the chest are the most frequent seat of inflammations, dropsies, &c.; whilst in that bordering on old age the inferior surfaces like the peritoneum, are the more often affected.
In old age, the serous system becomes dense and compact; its adhesions to the neighbouring parts are more evident; thus it is less capable of the different locomotions of which we have spoken. Its forces, which are weakened, render absorption in it less easy; it is the frequent seat of dropsy. When it is affected with some diseases, its want of energy imparts to them a remarkable chronic character. There are many old people at the HÔtel Dieu with tubercular inflammations of the peritoneum, which they have had for a long time, whilst young persons are overcome much quicker by the same inflammations. Thus cancers in aged persons often remain almost inert, they frequently are not even painful, whilst their periods are usually most rapid in adults.
The serous effusions are more rare than the infiltrations of the sub-cutaneous cellular texture in old people; but they take place more commonly than those of the intermuscular texture.
The serous system becomes ossified, but not like the arterial, the cartilaginous, &c. from the natural effect of age. Its membranes are not constantly found osseous in old age, and when this phenomenon takes place, it happens at every age. It is a disease, a real tumour, whilst in the arteries and the cartilages it appears to belong to a natural series of functions. I have a preparation in which the arachnoides is evidently osseous in many points, and as it is upon the dura-mater that these productions are formed, their existence serves very well to prove that the arachnoides is distinct from it; for at the place where they are found, they can be easily separated from each other.
III. Preternatural Development of the Serous System.
I shall not repeat here what I have said, in speaking of the cellular texture, upon the formation of the different cysts. These cysts, completely analogous to the natural serous membranes, should be really considered as a preternatural development of these membranes in the economy; they have the same appearance and almost the same texture, furnish their fluid according to the same laws, and appear to be the seat of a constant exhalation and absorption; since they are in vain emptied by puncture, their fluids are always reproduced, until they are removed. For example compare the cyst which has grown preternaturally along the spermatic cord, with the tunica vaginalis filled with water in hydrocele. If the size of these tumours, which are often found together, is the same, it is impossible to discover any difference in their form, appearance, texture, properties, functions, &c.
