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.