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.