Part 29

This thickness takes from the epidermis the transparency it has in the other parts; it is whitish and opake even on the hand and the foot. Thus the epidermis which, in negroes not being coloured, allows the blackness of the subjacent reticular texture to be seen, conceals in part this blackness in these places. I have observed however, by means of maceration, that the less deep colour of the soles of the feet and the palms of the hands depends also in this race upon this, that the reticular texture is really less coloured. We might say that every thing relates to animal sensibility in this region, the capillary net-work of which appears to be less, and in which all the phenomena that are derived from organic sensibility are much less active.

In examining in this relation the hand and foot of a negro, I have been led to make upon the colour of the reticular body some other experiments, which form a short digression. 1st. By plunging into boiling water a piece of the dermis taken from any part it becomes twice as black, almost immediately; which is probably owing to this, that the fibres in approximating by the horny hardening, bring together the colouring particles, whence arises a deeper black. This phenomenon is very striking, when the piece plunged into the water is compared with one that has not been. 2d. Maceration for a month or two, sometimes removes the epidermis without the reticular body, the seat of colour, and sometimes detaches the whole together. 3d. Being immersed for some days in cold water produces no sensible effect. 4th. Long continued stewing hardly changes at all this colour, after the deep tinge that has been suddenly given to it. Only by scraping with a scalpel the external surface of the skin which is then reduced to a kind of gelatinous pulp, we easily detach the coloured reticular body from it, which however always remains adherent to a small portion of the chorion. 5th. Sulphuric acid, which reduces the skin like all the other organs to a kind of pulpy state, also enables us to remove this coloured portion easily, which is detached in separate pieces, but the shade of which is hardly altered at all. 6th. Nitric acid, though very much weakened, does not facilitate so much the removal of this coloured portion. It yellows the internal surface of the skin and the epidermis; but it has appeared to me to produce but very little effect upon the blackness of the reticular body. 7th. A portion of the skin of a negro, immersed for twenty-four hours in a solution of caustic potash, has not appeared to me to have undergone any alteration in its colour. I have made the same observation when I used a weaker solution. 8th. Putrefaction detaches the coloured portion of the skin, sometimes with the epidermis, sometimes alone, but it does not alter its colour. I have not employed other agents to ascertain the nature of this colour of the skin of negroes. Let us return to the epidermis, which we have for a moment lost sight of.

Where it is very thick, as on the concave surface of the foot and the hand, we see that it is evidently formed by layers added to each other, and which are separated with difficulty, because their adhesion is so intimate. Everywhere except in the foot and the hand, there is but a single layer; no fluid penetrates the epidermoid texture. Cut in different directions either in the living or dead body, it allows nothing to ooze through it. Their scales are always dry; no blood vessel exists in them. The absorbents and exhalants only pass through it without anastomosing, without winding on its interior before opening on its surface, as happens in the serous membranes, which on this account become black by injection, though but little blood appears to enter them during life. The epidermis on the contrary is never coloured by this means, even when the injection, being very fine and driven with success, oozes out on the external surface of the skin. Thus, in inflammation, in which all the cutaneous exhalants are full of blood which they do not contain in the natural state, this fluid never enters the epidermis, which is uniformly disconnected with all the diseases of the subjacent reticular body, and which, being only raised up by inflammation, is detached and afterwards renewed.

The epidermis has evidently no nerves. It is also destitute of cellular texture; thus fleshy granulations, which are formed by this texture, never arise from this membrane; the excrescences of which it is the seat have not the character of the different tumours which the cellular texture especially contributes to form, such as fungi, schirri, &c.

From this it is evident that none of the general systems common to all the organs, enters into the epidermoid system. It has not then the common base of every organized part; it is as it were inorganic in this respect.

The epidermoid texture exhibits no fibre in its interior; it has in general but little resistance, and is broken by a slight distention, except on the fingers and the hand where it resists more, on account of its thickness.

The action of the air hardly alters it at all. Only when it is exposed to it after having been removed in the form of a large layer, it hardens a little, becomes a little more consistent, and is torn with less ease. It is of all the organs, next to the hair and the nails, that which drying changes the least in the natural state. It also becomes a little more transparent by it; but resumes its ordinary state when again immersed in water, which proves that it contained a little of it in this state. The action of the air, which is so quickly efficacious upon the skin in putrefaction, leaves it then wholly untouched. It is only raised up, but does not itself putrify. Separated in this way and washed to cleanse it of the fetid substances that might adhere to it, it exhales no bad odour. Kept a long time in moist air, alone and well separated from the neighbouring parts, it does not alter. It is, next to the hair and the nails, the most incorruptible of the animal substances. I have preserved a foot found in a cemetery, the skin and fat of which are transformed into a fat, unctuous and hard substance, which burns in the candle, whilst the epidermis, which is very thick, is hardly changed at all in its nature.

The action of water upon the epidermis can be considered under many relations. 1st. During life it whitens it, when it is some time in contact with it, and at the same time wrinkles it at different points. We often see this phenomenon in the hands on coming out of a bath; but it is particularly evident after ten or twelve hours application of an emollient cataplasm, in which the action of the farina is nothing, and in which it is the water that produces the whole effect. This whiteness of the epidermis appears to be then owing to its having really imbibed some of the fluid. It is the same phenomenon that takes place on the serous, fibrous, membranes, &c. which, having become artificially transparent by drying, whiten again when immersed in water. Here the epidermis, naturally transparent, whitens by the addition of this fluid. In this state it renders the sensibility of the papillæ infinitely more obtuse; I have often made the experiment upon myself, by applying a cataplasm in the evening and removing it the next morning. When the water is evaporated which the epidermis has imbibed, it again becomes transparent, wrinkles, resumes its natural state, and allows the sensibility of the skin to be again apparent. This phenomenon is especially observed upon the epidermis of the foot and the hand, for it is not often as sensible elsewhere. 2d. In the dead body, the epidermis separated from the skin, and immersed in water, whitens also, but does not wrinkle. Left to macerate in water, it does not undergo any putrid alteration. Only there rises on the surface of the fluid many particles, which being in juxta-position, form a whitish pellicle of the nature of which I am ignorant. At the end of two or three months, the epidermis thus left in water, softens, does not swell, and is torn with great ease; it is not reduced to a pulp analogous to that of the other organs thus macerated. 3d. When stewed, the epidermis does not undergo, at the instant of ebullition, a horny hardening like all the other organs. Hence why, whilst by this horny hardening the skin is much diminished in extent, the epidermis which remains the same is obliged to be folded in different directions. When the ebullition is continued, this membrane becomes less resisting and breaks with great ease, but is never reduced to gelatine, does not acquire a yellowish colour, and does not become elastic like the organs which furnish much of this substance; besides, we know that the epidermoid texture does not combine with tannin, and that it is even an obstacle to it when it tends to penetrate the skin. After long stewing, the different layers which compose the epidermis of the palm of the hand, and especially that of the sole of the foot, are separated with great ease; this is the best way of seeing this lamellated structure. Between these layers there is often formed on the foot small vesicles filled with serum.

Caloric produces upon the epidermis phenomena wholly different from those which the other systems experience from the contact of this substance. A portion of this membrane well dried by the action of the air, and exposed to the flame of a candle, 1st, does not undergo hardly at all the horny hardening, as a portion of skin does when thus treated; 2d, it exhales a fetid odour analogous to that of burnt horn, and different from that of all the other textures when subjected to the same experiment; 3d, it burns with great ease, which does not take place with any of the preceding systems when dried; it is often even sufficient to put the fire to it at one end to consume it entirely; 4th, at the place of the flame we see a blackish bubbling fluid, from which often escapes little burning drops, and which is very analogous to that of a feather when burnt. It is evidently an oil which supports the combustion by its great abundance, and which does not appear to be found in as great quantity as in the hair and the nails. This oil deserves particular examination; it is that which gives out in burning so disagreeable an odour, and which forms those burning and whitish drops of which we have spoken. It appears to be of the same nature as that which Bertholet obtained from the hair in so great a proportion. After combustion there is left a blackish charcoal, which I have not analyzed.

Light does not appear to have a great action upon the epidermis, which I have found of the same colour, in portions of skin blackened by it, and in those which have been sheltered from it.

Nitric acid yellows very sensibly the epidermis, more even than any other animal substance; but it does not dissolve it without great difficulty. The sulphuric on the contrary acts very powerfully upon it, especially when it is a little concentrated. When it is drawn out after having been a short time plunged in, it is found to be very thin, extremely transparent, and almost similar in this respect to the pellicle that is taken from an onion. This curious phenomenon has often struck me. When left too long in the acid, the epidermis is finally entirely dissolved in it.

The alkaline lies dissolve this membrane, but with difficulty. Pure alkali has a very prompt action upon it.

Alcohol has no influence upon the epidermis.

III. _Properties._

The epidermis has but very little extensibility, since the least cutaneous tumour can tear it and raise it up in scales, as in herpes, or in larger pieces, as from a blister. Yet it is not entirely destitute of it, as the vesicle proves which is formed by this last. Its contractility of texture is nothing. We observe, that when no longer distended, this bladder remains flaccid and never contracts.

Every kind of animal sensibility is foreign to the epidermis. We know that it can be pricked, cut or torn, without being felt. It is especially on the palms of the hands and the soles of the feet that these experiments are easily made. The thickness of this membrane is such in this place, that we can remove layers of it, as we see done by those who try the edge of an instrument, that it is possible even, as most cooks do, to put it in contact with live coals, and that it is not impossible to carry a red hot iron over it. It is by this insensibility that it blunts the action of the acids, the caustic alkalies, and of all the powerful stimuli, which when in contact with the dermis laid bare by a blister, give excessive pain.

The epidermis differs from all the other organs that are destitute, like it, of animal sensibility, as the cartilages, the tendons, the aponeuroses, &c. in this, that it is never capable of acquiring it; whereas the others, if a little excited, often take a degree of it superior to that of the organs which naturally possess it. Whence does this arise? From the fact, that in order that the animal sensibility may arise in an organ it is necessary that the rudiments of it should be there already, and that this organ should enjoy organic sensibility, which, when raised by irritation is transformed into animal; now the epidermis appears to be destitute also of this last property, as well as of insensible contractility. In fact, 1st, there is no sensible circulation in it. 2d. The exhalants and absorbents which go through it, are wholly foreign to it. 3d. No morbid phenomenon, that supposes organic sensibility, appears in the epidermis. It does not inflame; it is passive in all cutaneous affections, and never partakes of them notwithstanding its continuity. The impossibility of inflaming makes it an obstacle, wherever it exists, to cutaneous adhesions, which cannot take place until it is removed. Its internal surface, raised by a blister, and reapplied to the dermis by the evacuation of the serum of the vesicle by means of a small puncture, never unites again. 4th. The excrescences of which it is the seat, as corns, some indurations, &c. are inert and dry like it, and without internal circulation; if they are painful, it is from the pressure upon the subjacent nerves, and not from themselves. 5th. No sensible operation is performed in the epidermis; it is worn incessantly by friction, like inorganic bodies, and it is afterwards reproduced.

This continual destruction of the epidermis has not sufficiently arrested the attention of physiologists. The following are the proofs of its reality; 1st, if with the blade of a knife, we scrape strongly its external surface, a large quantity of powder is removed which sulphuric acid easily dissolves, and which is greyish. The epidermis whitens a little in this place, then resumes its colour, especially if it is moistened. By scraping again, we do not remove any more powder, it is necessary in order to obtain it, to wait twelve or twenty hours. 2d. This substance becomes superabundant, when the skin has not been washed for a long time. Hence why those who soak their feet that have not been cleaned for a long time, detach so great a quantity of it. It is especially on the sole of the foot that this substance is formed in abundance. We often observe in dead bodies that it forms almost a layer in addition to the epidermis, but which is very distinct from it, and can be removed with ease. I attribute this circumstance to the thickness which the epidermis has in this place. We should no doubt find as much upon the hand, were it not for the continual friction of this part. We see it often on the patients in hospitals, after remaining a long time in bed without having been cleansed.

Water naturally removes this substance, that is produced by the destruction of the epidermis, and, which, mixing with the residue of transpiration, that the air cannot carry off by evaporation, renders bathing, as I have observed, a natural want. Though it may be neither exhaled nor absorbed, and though its production may appear to be owing to mechanical friction, yet we can, in its relation, consider the epidermis as an emunctory of the body, since it is renewed by a substance coming from the dermis, as fast as it is removed.

It is evident, as the epidermis has no vital properties, that it cannot be the seat of any kind of sympathies, which are aberrations of these properties. Hence its life is extremely obscure, I doubt even if it possesses vitality. We might almost say that it is a semi-organized body, inorganic even, which nature has placed between external inanimate bodies and the dermis, which is completely organized, in order to assist their passage and guard against their force.

The epidermis has a property very distinct from those of most of the other systems; it is that of being reproduced when it has been removed. It grows anew and is formed again with an appearance exactly similar to what it first exhibited; it is that which makes it differ from all the other systems, as the cellular, which throw out vegetations when they are laid bare, but which are only reproduced in an irregular manner, and wholly different from their natural state. How is the epidermis thus reproduced? Is it the pressure of the atmospheric air which renders the external surface of the skin callous? Is it the air, which, by combining with the products which escape from this surface, forms a new compound? I know not. What is certain is, 1st, that this production is wholly different from that of the internal organs; 2d, that it cannot take place except upon the skin, and that the fine pellicle that covers all the other cicatrized parts, after a wound with loss of substance, does not resemble it at all and presents even a texture wholly different. Thus this pellicle is not raised up by the different means which raise the epidermis; thus it often becomes the seat of acute sensibility which is never the case with the epidermis. This is what takes place especially in changes of weather, a time in which the cicatrices become, as we know, very painful; I have often observed, that not only the interior, but the pellicle even of the cicatrix are then sensible. Besides, when this pellicle is formed, red blood vessels evidently penetrate it, whilst nothing similar is observed in the formation of the epidermis.

It is this faculty of reproduction which is put in action in many epidermoid excrescences, as in corns, and callosities which have nothing in common but the name with those which form the edge of fistulas, &c. All these excrescences are insensible, without vessels or nerves, of the same consistence and the same colour as the epidermis; they are often removed from it and afterwards formed again. It appears that external pressure has much influence upon their development; too narrow shoes and the solid bodies which are used on the hands of smiths and other workmen are the frequent cause of them.

I preserved a great part of the skin of a man who died at the Hôtel Dieu, and his epidermis, which was treble the thickness from his birth and even in the womb of his mother, that it is in the ordinary state, had been subject during his life to a continual desquamation which made the whole of it appear as if covered with herpes, though nothing similar to this affection existed upon the dermis, which was perfectly sound. The face alone was exempt from this defect of conformation.

The epidermis is not only reproduced when the whole of it has been removed, but also when the superficial layers alone have been taken away, especially on the foot and the hand on which other layers arise upon those which the cutting has laid bare; which proves that they are not, as has been said, the juices of the reticular body which form it by drying.

IV. _Development._

Those who have thought that the epidermis is formed by pressure, would be convinced that this is not the case if they would examine that of the fœtus, which is very distinct, more even in proportion than many other systems. We observe it when the skin begins to leave the pulpy state of which we have spoken. At the end of the fifth month, it has proportions analogous to those which it will afterwards exhibit. It is very thick on the soles of the feet and the palms of the hands, and very thin elsewhere; it is easily detached by all the means we have pointed out. We know that in a fœtus that has died and become putrid in the womb, it is found in great measure detached. At the place of the umbilical cord, it is continued in an insensible manner with the skin.

At birth, though it is in contact with a fluid that is new to it, it does not undergo a great alteration; which proves that the air has little or no agency in its formation. It becomes thicker as we advance in age, and follows, in this respect, nearly the same proportions as the skin. Beyond the twenty-sixth or thirtieth year it increases no more. I have often raised up in many places the epidermis of an old person; it has not appeared to me to differ much from that of the adult; it is a little more subject to scale off and it is a little thicker. In some miserable objects which come to hospitals, there is often vermin in cracks of the epidermis, whose layers are afterwards separated by them and in which they live; so that I have seen the epidermis in this way conceal thousands of little animals, which were evidently found between the layers of this membrane, and which were not upon the reticular body and the papillæ. It is the only means that has shown to me the lamellated structure of the epidermis, in any other place than on the foot and the hand, in which I have never seen vermin.

The cracks of the epidermis in old age appear to arise from its dryness owing to the want of exhalation; it is that which renders the skin so rough and harsh. What contributes to it also is, that as it has many inequalities on account of its numerous folds, frictions being more felt in these prominent places, make the epidermis scale off; thus in the adult the same cause renders it scaly on a tubercular skin, whilst a skin that is smooth and well distended with fat, undergoes every kind of friction without desquamation.

ARTICLE SECOND.

INTERNAL EPIDERMIS.

All authors have admitted the epidermis of the mucous membranes. It appears that most have believed that it is only this portion of the skin which descends into the cavities to line them. Haller in particular is of this opinion. But the slightest inspection is sufficient to show, that here as upon the skin, it forms only a superficial layer over the papillary body and the chorion. Boiling water which detaches it from the palate, the tongue and the pharynx even, enables us to see the two other layers.

I. _Epidermis of the origin of the Mucous Surfaces._

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.