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.