The epidermis is very distinct upon all the origins of the mucous system, upon the glans, the entrance of the anus, the urethra, the nasal fossæ, the mouth, &c. It is demonstrated in these places by the excoriations that take place there, upon the lips especially, by dissection with a very fine lancet, by the action of boiling water, maceration, putrefaction and even epispastics, as is proved by the fact that the ancients employed this method to make the edges of a hare-lip raw. The delicacy of this epidermis is much greater than on the skin; and as it is more in the interior this delicacy increases. It is to this circumstance that must be attributed the ease with which different remarkable modifications are produced through this membrane, when in galvanic processes, we arm with zinc the surface of the tongue and with another metal the mucous surface of the conjunctiva, the pituitary membrane, the surface of the rectum, the gums, &c. and bring in mediate or immediate contact these different metals.
The mucous epidermis is quickly reproduced when it has been removed. Destitute of every kind of animal and organic sensibility, it is in this respect, destined like the skin, to defend the very sensitive papillary body that is subjacent to it. It is to its presence upon the mucous membranes, that should be in part attributed the faculty they have of being exposed to the air, and even to the contact of external bodies, without exfoliating or inflaming as in preternatural anus, prolapsus of the rectum, &c.; whilst the serous membranes cannot bear this contact with impunity.
Besides, the nature of the mucous epidermis is the same as that of the cutaneous. Submitted to the action of the same agents, it gives the same results. The excrescences formed on its surface are also analogous, though much more rare. It becomes callous by pressure. Chopart relates the case of a shepherd, whose urethra became so, from frequently introducing a small stick to procure pleasure. We know the density that this covering has in the stomach of the gallinaceous animals, and in certain cases in which the mucous membranes come out of the body as in prolapsus of the rectum, the vagina, the womb, &c. Sometimes in those cases the pressure of the clothes produces in this epidermis a thickness evidently greater than what is natural to it; it is this which then makes these membranes lose in part the bright red that characterizes them in the interior.
II. Epidermis of the deep seated mucous surfaces.
The epidermis gradually becomes more delicate, and is soon almost insensible, on the internal mucous membranes. 1st. In the stomach, the intestines, the bladder, the gall-bladder, the vesiculæ seminales, in all the excretories, &c. the most delicate instrument cannot raise it up. 2d. In the maceration and ebullition of the mucous system of these parts, I have never seen the epidermis raised up on its surface. 3d. I have drawn out of the abdomen of a dog a portion of intestine; its mucous coat has been laid bare by an incision, and I have applied an epispastic to it; more redness was seen upon the free surface of this coat, but no pellicle was raised up from it. 4th. We do not see in preternatural anuses, complicated with inversion, excoriations analogous to those of which the surface of the lips, that of the glans, &c. are the seat. 5th. I have already had frequent occasion to open bodies affected with acute or chronic catarrhs of the intestines, the stomach, the bladder, &c.; now I have never seen the epidermis separated by inflammation, as happens after erysipelas, phlegmon, &c. upon the cutaneous organ. 6th. We do not see upon the deep seated mucous surfaces those exfoliations, desquamations, &c. so frequent upon this after many affections.
From all these considerations it would appear, that the epidermis does not exist upon the deep seated mucous surfaces, and the great quantity of mucous juices constantly poured out by the subjacent glands, supplies its place in defending the papillæ and the chorion from the impression of substances heterogeneous to the economy, contained in the internal cavities. Yet there is a circumstance that would seem to demonstrate the existence of the epidermis upon the deep seated mucous surfaces; it is the separation of preternatural membranes, which are often detached from these surfaces, and which may be considered as a kind of epidermoid exfoliation. Many authors give examples of these membranes formed either upon the bladder and voided by the urethra, or upon the stomach and œsophagus and thrown up by vomiting, or upon the intestines and expelled with the alvine evacuations; Haller has collected many cases. Dr. Montaigu informed me that he saw a membrane vomited up, which formed a sac without a rent, exactly analogous to that of the stomach whose internal surface it lined. Desault saw a sac almost analogous to the bladder, voided by a patient who was affected with retention of urine.
I confess that I have made no observation on this point, so that I cannot say what is the nature of these membranes. But authors in general agree in attributing to them a soft and pulpy nature, which does not appear to me to accord with that of the epidermis. I have many times seen at the Hôtel Dieu white membranes detached from the œsophagus after poisoning with the nitric acid. But these membranes are evidently the superficial portion of the mucous organ, which is disorganized, and thrown off by suppuration which takes place below. It is thus that cutaneous eschars fall off in the form of membranes from large burns; in this way the osseous layers are formed in necrosis, which are only the superficies of the bone that dies and is detached in a lamellated form.
From this, the existence of the epidermis of the deep seated mucous surfaces appears to me to be very uncertain, and cannot be admitted till a new examination, which will, I think, prove rather against than in favour of its existence. What is the place in which the epidermis terminates that lines the origin of the mucous surfaces, or if it exists everywhere, where does it begin to become no longer apparent from the action of our different reagents? We cannot, I think, determine with precision; it diminishes in an insensible manner, and is lost as it were by degrees.