12. The free surface of mucous membranes, or that which is continually moistened by the fluid from which they borrow their name, presents two kinds of wrinkles or folds, the one inherent in their structure and which is constantly present, whatever may be their state of contraction or dilatation, such as the pylorus, the valvula conniventes, the valve of the colon, &c. These folds are formed, not merely by the mucous membranes, but also by the intermediate membrane mentioned above, and which in these parts takes a remarkable density and thickness.
13. The other folds may be called accidental, and are only observed during the contraction of the organ; such are those of the inner surface of the stomach, and of the large intestines, &c. In most of the human subjects brought to our amphitheatres, these folds in the stomach, of which so much has been said, are not perceptible, because generally the subject has died of a disease which has impaired the vital powers, without preventing all the action of this viscus; so that, although it is frequently found empty, its fibres are not in the least contracted.
14. In experiments on living animals, on the contrary, these folds are very apparent; and observe how they may be demonstrated. Let a dog eat or drink copiously; open it immediately, and make an incision into the stomach the whole length of its greater curvature, no fold will then appear, but it soon contracts, its edges are drawn in, and the whole of the mucous surface is covered with numerous prominent plicæ in the form of circumvolutions. The same result may be observed in the stomach of a recently killed animal by distending it with air, and then opening it; or, what is still better, by laying it open whilst empty, and stretching it, the folds will disappear, and when we cease to make the extension they immediately form again and are very apparent.
15. I would observe on the subject of inflating the stomach, that by distending it with oxygen gas the application of this fluid does not produce more prominent folds, and therefore no stronger contraction, than when carbonic acid gas is used for the same purpose. This experiment presents a result very similar to what I have observed when I have rendered animals emphysematous by different æriform fluids. Frogs and Guinea pigs (these are the two kinds I have chosen, the one being an animal of red and cold, and the other of red and warm blood) presented very little difference in their irritability, or their Galvanic susceptibility, whether inflated with oxygen gas or with carbonic acid gas. They live very well with this artificial emphysema, which gradually disappears. Inflation with nitrous gas is always mortal, and its contact appears to strike the muscles with atony. The stomach distended with it very soon loses its power of contracting, and its folds disappear. Here, as in all the experiments which have the vital powers for their object, we frequently obtain very variable results.
16. It follows, from what we have said respecting the folds of mucous membranes, that in the contraction of the hollow organs, which are lined by them, they suffer but a very trifling diminution of surface, they scarcely contract at all, but fold themselves within; so that in dissecting them upon their contracted organ, we have an extent of surface nearly equal to that which they present during its dilatation. This assertion, which is true concerning the stomach, the œsophagus, and the intestines, is, perhaps, not quite so as respects the bladder, whose contraction does not show within such prominent folds, but they are sufficiently marked to bring the mucous membrane of this organ under the general law. It is, also, nearly the same with the gall bladder; yet we find here another cause; observed alternately, in a state of hunger and during digestion, it will be found to contain double the quantity of bile in the former case that it does in the latter, as I have had the opportunity of seeing in numerous instances, in experiments made with this object in view, or with other intentions. Now, when it has evacuated part of its contents it does not contract upon the remainder of the bile, with the energy of the stomach when it contains but little food, nor with the power of the bladder when it contains but a small quantity of urine, but is then flaccid, so that its distention or nondistention has but very trifling influence upon the folds of its mucous membranes.
17. Moreover, in saying that the mucous membranes present with trifling variation the same extent of surface in the dilatations as during the contraction of their respective organs, I intend to speak of the ordinary state of the functions only, and not of those enormous dilatations which are frequently seen in the stomach and bladder, more rarely in the intestines. In such cases there is doubtless a real extension, which in the membrane coincides with that of the organ.
18. One remarkable observation that the free surface of mucous membranes affords us, and which I have already pointed out, is, that this face is everywhere in contact with bodies of a different nature to that of the animal: these bodies are either introduced from without for its nourishment, and are not yet assimilated to its substance, as we see in the alimentary canal and in the trachea, or they are produced within, as we observe in the excretory ducts of the glands, which all open into cavities lined by mucous membranes, and discharge those particles, which, after having for some time formed a part of the composition of the solids, become heterogeneous to them, and are thrown off by that habitual action of decomposition, which takes place in living bodies. According to this observation we must consider the mucous membranes as defensive coats, placed between our organs and foreign bodies, and that they consequently serve the same purpose internally which the skin does externally, as respects bodies that are in contact with it.
SECTION III.
OF THE INTERIOR ORGANIZATION OF MUCOUS MEMBRANES
19. Between the mucous and other membranes, as respects their interior organization, there is this essential difference, that they are always formed by several thin fibrous layers; these layers or coats are, with the exception of the rete mucosum, the same as those which compose the skin with which these membranes have the most exact analogy. We are about to examine separately each of these layers, which are the epidermis, the corps papillaire, and the chorion, in their general attributes; we shall afterwards consider the particular modifications which they undergo in the different parts of the mucous surfaces.
20. All authors have admitted the epidermis of mucous membranes: it appears, even, that the greatest part of them have believed that it is merely that portion of the skin which descends into the cavities to line them; Haller in particular is of this opinion; but the least inspection is sufficient to show, that here, as in the skin, it forms but a layer superficial to the corps papillaire and chorion; boiling water, which detaches it from the surface of the palate, the tongue, and even from the pharynx, leaves the two other coats denuded and apparent.
21. This epidermis is very distinct upon the glans, at the anus, at the orifice of the urethra, at the entrances of the nasal fossæ, and of the mouth, and in general wherever the mucous membranes arise from the skin. It is demonstrated in these different places by the frequent excoriations which occur on them; it may be raised from the lips by a very fine lancet by the action of boiling water, a hot iron, or even by epispastics, as the method of the ancients proves, who employed them to produce a fresh raw surface for the cure of the hare lip.
22. But in proportion as we go into the depth of the mucous membranes, the existence of this coat becomes more difficult to be demonstrated; it cannot be raised by the finest instrument, nor detached by boiling water, at least in the gall bladder, in the stomach, and intestines. I have made these experiments in fresh slain animals, and also in those where the natural heat had quite left them. But what our experiments cannot effect, inflammations will often produce. All the authors, who have written on the affections of the organs which are lined by these membranes, mention instances in which flakes, more or less considerable, have been voided by the urethra, anus, mouth, nostrils, &c. Haller has collected a great number of similar observations. Without