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.
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.