applied to the external face of the skin between the chorion and the
epidermis, pierced with an infinite number of openings through which
the papillæ pass. I do not know how we can demonstrate this layer,
which escapes according to the opinion of most of them, when the
epidermis is detached. In order to see it I have employed a great
many means, but no one has succeeded. 1st. Such is the adhesion
of the epidermis to the skin, that in a sound state we can hardly
separate them without injuring one or the other. Yet with the greatest
precaution we see nothing mucous on the chorion when it is laid bare.
2d. A portion of skin cut longitudinally, especially from the foot
where the epidermis is very thick, allows us to see very distinctly on
the divided edge the boundaries of this and of the chorion; now nothing
escapes from about the line which separates them. 3d. In ebullition in
which the epidermis has been removed, nothing remains upon the internal
surface, nor upon the chorion. 4th. Maceration and putrefaction, the
latter especially, produce upon the chorion a kind of glutinous layer
the instant the epidermis is removed. But this layer is entirely the
product of decomposition. Nothing similar is met with in the ordinary
state.
I believe, from all these considerations, that there is not a substance
deposited by the vessels upon the surface of the chorion, extravasated,
stagnant upon this surface, and representing there a layer in the sense
in which Malpighi understood it. I believe that we ought to understand
by the reticular body, a net-work of extremely fine vessels, whose
trunks already very delicate, after having passed through the numerous
pores with which the chorion is perforated, come and ramify upon its
surface, and contain different kinds of fluids.
The existence of this vascular net-work is placed beyond a doubt
by fine injections which change the colour of the skin entirely
externally, without altering it much within. This is, as I have
observed, the principal seat of the numerous eruptions most of which
are really foreign to the cutaneous chorion.
We may then consider the reticular body as a general capillary system,
surrounding the cutaneous organ, and forming with the papillæ a layer
between the chorion and the epidermis. This system contains in most
men, only white fluids. In negroes, these fluids are black. They have
an intermediate tinge in the tawny nations. We know how much the shades
vary in the human race. Hence the colouring of the skin resembles
nearly that of the hairs, which evidently depends upon the substance
existing in their capillary tubes; it is analogous to that of the
marks at birth, that are commonly called nævi materni, and in which we
never see a layer of fluids extravasated between the epidermis and the
chorion.
Moreover, I think we know but little as yet concerning this substance,
which fills a part of the external capillary system. It does not
circulate in it, but appears to remain there till another replaces it.
When we examine the skin of a negro, we see a black teint, and that is
all. In maceration I have observed that this teint is sometimes removed
with the epidermis, and that it sometimes remains adhering to the
chorion. It is very evidently foreign to both, since both have the same
colour in whites as in blacks. It is never reproduced, after it has
been removed; for cicatrices are white in all people.
Is there in white people a white substance which, remaining in the
external capillary system, corresponds to that of negroes, or does the
colour of their skin depend only upon the epidermis and chorion? I
have been tempted to believe that they also have a colouring substance,
since the long-continued action of a powerful sun evidently blackens
them. This circumstance has even made me believe that whiteness is
natural to all men, and that there was but one primitive race which has
degenerated according to different climates.
But in order to be convinced of the diversity of races, it is
sufficient to observe, 1st, that the teint of the skin is but one of
the characters which distinguish each race, and that many others are
always united to it. The nature and form of the hair, the thickness
of the lips and the nose, the width of the forehead, the degree of
inclination of the facial angle, the whole appearance of the face, &c.
are constant attributes which indicate a general modification in the
organization, and not merely a difference of the dermoid system. 2d.
White people become tawny in hot countries; but they never acquire the
teint of the people of the country. 3d. Removed to cold countries in
early age, or even born in them, the blacks always remain so; their
shade hardly changes at all from generation to generation. 4th. Colour
by no means follows temperature exactly; we see many varieties in the
shades of people who live under the same degree of latitude, &c.
Every thing proves then that the colour of the skin is but an insulated
attribute of the different human races, though it is that which is most
striking to our senses, and that we should not attach to it a greater
importance than to many others which are drawn from the stature, which
is oftentimes very small, as in the Laplanders, from the broad and
flat face, as in the Chinese, from the dimensions of the chest, of the
pelvis, the extremities, &c. It is from the differences of the whole,
and not from those of an insulated part, that the lines of demarcation
should be made which separate the races. The European face and forms
are in general the type with which we compare the exterior of the
other nations. The ugliness or beauty of the human races are, in our
way of considering it, measured by the distance which separates these
races from ours. Such is in fact the force of habit with us, that we
rarely judge in an absolute manner, and that every object which is much
removed from those to which we are accustomed, is disagreeable to us
and sometimes even disgusting.
Besides, the colouring matter of the cutaneous reticular body is more
interesting to the naturalist than to the physician. What should
particularly arrest the attention of the latter is the portion of the
capillary system exterior to the skin in which the fluids circulate.
In fact, besides the portion which is the seat of colour, there
is evidently another that the white fluids constantly pervade, in
which they are moved with more or less rapidity, and in which they
continually succeed each other. It is from this portion that the
exhalant pores arise which furnish the sweat; it is this vascular
net-work which is the seat of erysipelas and of all the cutaneous
eruptions that are foreign to the chorion.
The blood does not penetrate it in an ordinary state, but a thousand
causes can at every instant fill it with this fluid. Rub the skin
briskly, and it reddens in a moment. If an irritant is applied to
it, whether it acts mechanically like nettles, the appendices of
which penetrate the epidermis, or exerts a chemical action, like the
frictions with ammonia, or the action of fire when a portion of skin is
held too near it, instantly the sensibility of this vascular net-work
is raised; it invites into it the blood which it formerly repelled;
every part of a surface reddens in proportion to the irritation. If
passion acts powerfully upon the cheeks, immediately a sudden redness
is evident in them. All rubefacients exhibit moreover a proof of the
great tendency which the sensibility of the superficial capillary
system of the dermis has to place itself, if it be ever so little
excited, in relation with the blood which in the ordinary state is
foreign to it.
Vesicatories depend upon the same principle. Their first effect is to
fill with blood the cutaneous capillary system, where they are applied,
to produce in it a sudden erysipelas, and then to occasion a copious
serous exhalation under the raised epidermis. They effect in a few
hours what most cases of erysipelas do in many days; for we know that
most of them terminate by vesicles which are raised above the skin.
In burning, carried sufficiently far to be more than a rubefacient,
and yet not so as to produce the horny hardening, there is also a
sudden increase of exhalation under the raised epidermis. In general
the production of every cutaneous bladder is always preceded by an
inflammation of the external surface of the skin. This phenomenon is
not exclusively confined to this system. We have seen the serous, as
soon as it is laid bare and irritated considerably, redden in a short
time by the passage of the blood into its exhalants; which constitutes
an inflammation to which often succeeds a copious exhalation of milky
or other kind of serum. This exhalation does not remain upon the
surface, and does not form vesicles there, because it has no epidermis;
this is the only difference between these phenomena, which at first
view do not appear to be the same in the serous and cutaneous systems.
It is not only the irritation of the cutaneous organ which makes the
blood pass into the external capillary system. Whenever the heart is
powerfully excited and it accelerates the course of this fluid, it
always tends to go into it; this is what is evidently seen, 1st, after
violent running; 2d, in the hot period of a paroxysm of fever.
Upon this subject I will make a remark which appears to me to be very
important; it is that the capillary system of the face is, more than
that of all the other parts of the skin, exposed to be thus penetrated
with blood. 1st. This is evident in the two cases of which I have just
spoken, and in which the action of the heart is increased. 2d. In the
passions, the skin remains the same in the other parts, whilst that
of the face suddenly becomes pale or red. 3d. We know that physicians
frequently examine the state of the facial capillary system, which is
almost always affected by the state of the internal viscera, and is
full of blood or empty, according as it is sympathetically influenced.
4th. In various kinds of asphyxia, in those especially produced by
submersion, by the vapour of charcoal, by strangulation, &c. the face
is uniformly of a violet colour from the passage of the black blood
into its external capillary system, into which it is brought by the
arteries. Oftentimes the neck and the upper part of the chest are also
livid; but there is never a discoloration of the inferior parts. 5th.
In many diseases, in which death takes place by a kind of asphyxia,
because the lungs are the first interrupted, the dead bodies have a
violet-coloured and swollen face; this may be easily observed by all
who frequent dissecting rooms. There are a hundred subjects in which
the head has this lividity, to one in which it is observed in the
inferior parts. 6th. Most cases of apoplexy produce the same lividity
of the face.
To what is this extreme susceptibility of the facial capillary
system to admit the blood owing? Three things, I think, principally
contribute to it. 1st. The course is already opened to this fluid,
since the redness of the cheeks necessarily supposes its presence in
them, it only increases in quantity; whereas when another part of the
dermoid surface becomes red, all the blood which enters it is almost
accidental. 2d. The anatomical arrangement of the capillary system is
more favourable to this passage there than elsewhere; for it appears
that the communications of this system with the arteries of the chorion
are more free. What proves this is, that in injections the face is
coloured with great ease. There is undoubtedly no anatomist who has
not been struck with this phenomenon, especially in children, in whom
if the coarse injections of our dissecting rooms succeed at all, the
face becomes wholly black, whilst the fluid penetrates but very little
into the other parts of the cutaneous system. 3d. It appears that
there is a greater sensibility in the face; in fact the same irritant
brings blood there, which does not make it flow to any other place. For
example, a blow equal to a box on the ear does not redden the skin of
the arm, whilst it suddenly inflames the cheeks.
The blood disappears from the facial capillary system as it enters
it; in an instant the passions will successively produce there the
bright red of a paroxysm of fever, the whiteness of syncope and all the
intermediate shades. It is even the extreme ease with which this fluid
penetrates this system, that renders the face well adapted to serve as
a kind of picture, which the passions paint by turns with a thousand
shades, that are effaced, altered, modified and return again according
to the state of the mind.
I would observe upon this subject that the passions have in the face
three means of expression; 1st, the capillary system, a means wholly
involuntary, and which often betrays what we wish to conceal; 2d, the
muscular motion, which, by contracting or expanding the features,
expresses the melancholy or gay emotions, and to which belongs as
effects, the various wrinkles of which we have spoken; 3d, the state
of the eye, an organ, which, as Buffon has remarked, not only receives
the sensations, but also expresses the passions. The two last means are
to a certain extent voluntary; we can at least disguise them; whereas
we cannot deceive by the first. The actor imitates anger, joy, &c.
because we can give these passions by contracting the eye-brows, by
dilating the face in laughing, &c. But it is the rouge of the actress
that imitates modest chastity; it is by removing this rouge that she
imitates the paleness of fear, horror, &c.
I will add another essential observation in respect to the facial
capillary system; it is that it appears that its tendency to receive
blood, disposes it to become the more frequent seat of many affections.
We know, 1st, that erysipelas is much more frequent in this than in
other parts; 2d, that the variolous pustules are remarkably conspicuous
here; 3d, that many eruptions are more abundant here than elsewhere.
From all that we have just said, it is evident that it is necessary
to distinguish two portions in the capillary system exterior to the
chorion. 1st. One is constantly filled with the colouring substance of
the skin, a substance which appears to be stagnant like that of the
hair of the head, and that of the hair of the body, which is subjected
only to the slow and insensible motion of composition and decomposition
and which never exhibits that sudden increase or diminution of which we
have just spoken. 2d. The second is constantly pervaded by many fluids
which continually succeed each other there, and which constantly escape
by transpiration, and which are replaced by the blood, that insinuates
itself into this portion of the capillary system. These two portions
are entirely independent, and have probably no kind of communication.
It appears that at the instant of death there remains a certain
quantity of the white fluids in the second portion of the exterior
capillary system; the following experiment, which I have frequently
made, proves it; by plunging a portion of skin into boiling water, and
leaving it there an instant, the epidermis is raised up, not as a whole
as in a blister, but in an infinite number of small vesicles which are
formed suddenly on its surface, and which contain a serous fluid, that
escapes as soon as we open these vesicles.
_Papillæ._
We call by this name those small eminences that arise from the external
surface of the chorion, and which, piercing the capillary net-work
of which we have just spoken, become by their extremities contiguous
to the epidermis. These eminences are very evident in the palm of the
hand and the sole of the foot, where they are regularly arranged, in
the form of small curved striæ in different directions. We see them
through the epidermis, notwithstanding its thickness in these places.
But they are seen especially when this has been in any way removed,
as by maceration, ebullition, &c. If we cut longitudinally a portion
of the chorion of the foot, with its epidermis adhering to it, we see
between them along the divided edge, a line in the form of a curved
thread, which arises from these small eminences placed at the side of
each other.
In some other parts of the skin, we distinguish the papillæ in a very
evident manner; but in a great number, the epidermis being removed,
we see only a surface, slightly uneven from some small eminences,
especially towards the orifices through which the hairs and the vessels
pass, but we do not discover those regularly arranged eminences, the
papillæ properly so called.
We must not mistake for them the numerous and very evident prominences,
which render the skin of some subjects extremely rough. These
prominences are formed by small cellular, vascular or nervous bunches,
by sebaceous glands, &c. which are found near the small openings by
which the chorion opens under the epidermis, and usually transmits the
hairs. These bunches, lodged in the small oblique canals which are
terminated by these openings, raise the external side of them and thus
form this prominence. The following very curious experiment proves
this arrangement; when the skin is macerated for two or three months,
or even less, on the one hand these little bunches in which there
is almost always a little fat, are changed into that white, thick,
unctuous matter, analogous to spermaceti, into which fat kept a long
time in water is always converted; and on the other, the foramina
enlarging, as we have seen, and the skin changing into a kind of pulp,
we can easily remove it all around these little prominences, and see
that they are continued with the fat which fills the meshes of the
subjacent chorion, and which is also changed into a hard matter.
Injections have evidently proved to me that there were vessels in
these cellular bunches, and I have been convinced of it for some time
past by the dissection of some subjects that died of scurvy, in whom
the spots commenced by very small ecchymoses, similar as it were to
flea-bites, and which occupied these little eminences. The petechiæ
of adynamic fevers have a different appearance; but they belong also
to an extravasation of blood in the cellular texture, occupying the
small pores which open on the exterior of the chorion to transmit the
vessels, the hairs, &c. The more prominent these eminences are, of
which we have spoken, the more uneven is the skin. In general they are
more frequent on the extremities and on the back, than on the anterior
part of the trunk. In the extremities there are more of them in the
direction of extension, than in that of flexion.
We attach the idea of a beautiful skin, to that in which these small
tubercles are not found, and in which the chorion is united at its
external surface. Women have commonly this last arrangement more
evident than men. The epidermis which covers these eminences very
often scales off at that place, especially from strong friction, which
contributes still more to render the skin uneven, rough and harsh to
the touch where they exist, which might induce a belief that they are
formed by it, though it is always only accessory to them. Where it
is very thick, as in the palm of the hand and the sole of the foot,
it cannot be raised, and these small cutaneous tubercles are never
seen. In the face where many vessels pass from within outwards, by the
little pores of which we have spoken, we meet with hardly any more of
them. The papillæ scattered among these eminences, are in general very
slightly apparent in the places where they exist.
All anatomists attribute to these last a nervous structure; they regard
them as the termination of all the nerves that go to the skin, and
which, according to them, are expanded to form these, after having
first left their external covering. Some even say that they have traced
filaments even into the papillæ; I confess that I have never been able
to do it. In the ordinary state, the density of the chorion and the
extreme delicacy of the filaments, are evidently an obstacle to it. In
the state of long continued maceration, in which the chorion becomes
pulpy and in which we might consequently trace these filaments, were
it ever possible, it cannot be done. I do not however deny the texture
attributed to the papillæ. The acute sensibility of the skin seems even
to suppose it; but it is only analogy and not demonstration, which
establishes this anatomical fact; indeed all the other senses, whose
organs are so sensible, have the portion of them which receives the
impression of bodies continuous with a nerve.
_Action of different bodies upon the Dermoid Texture._
In most of the other textures, we have only considered this action in
the dead body, because during life, these textures always removed from
external bodies, cannot be influenced by them. Here we can regard it in
a double relation, since the skin is incessantly in contact with almost
all the bodies of nature.
_Action of Light._
Light evidently acts upon the dermis. Removed from its influence, men
are blanched, if we may so say, like plants. Compare the inhabitant
of a city, who is never exposed to the influence of the sun, with
the peasant who constantly is, and you will see the difference. It
appears that it is the light and not the heat which produces the effect
of which I have already spoken; for individuals who live in a warm
temperature, but removed from the solar light, become white like those
of cold countries. Thus we know that some men who keep their chambers
always very hot, are whiter than others who, living in a less hot
atmosphere, are constantly exposed to the sun. We might remain forever
in a bath of a temperature equal to the warmest seasons, and the skin
would not blacken. Apartments for study which are warmed with stoves,
and in which men remain as long as the labourer at his plough, are as
warm as the atmosphere of summer, and yet the skin of those who occupy
them never becomes darker. Besides an irresistible proof is that the
clothing which does not prevent the action of caloric upon the skin,
and which offers a barrier to the rays of light only, prevents the
cutaneous colouring that takes place upon the parts which the light
immediately strikes, as upon the hands, the face, &c.
I do not speak of the solar influence upon the vital forces of the
skin, as in cases in which sun-strokes produce erysipelas, or as when
light is employed medicinally to recall the life of a part; but it is
only in relation to the dermoid texture that I consider its action.
_Action of Caloric._
The action of caloric upon the skin exhibits very different phenomena,
according to the degree of it that it is applied.
1st. A warm atmosphere expands the dermoid texture, increases its
action, and makes most of the fluids which form the residue of
nutrition and digestion, pass off by the exhalants.
2d. When contracted by cold, this texture refuses to admit those
fluids, which then go off principally by the urine.
3d. The insensible change from one to the other of these two states,
does not disturb the functions. When this change is sudden, there are
almost always alterations in different organs, because the fluids
destined to pass out, cannot vary their direction as rapidly towards
this or that organ, as the cutaneous excitement produced by the sudden
changes from heat to cold.
4th. The skin resists a temperature much greater than that of the body;
it opposes an insurmountable barrier to the external caloric, which
tends to an equilibrium in animate as well as inanimate bodies. Thus
whilst these last are penetrated with this fluid in a medium warmer
than themselves, and soon acquire the temperature of this medium,
living bodies remain at the same degree, how much greater soever the
surrounding heat may be to their own. The curious experiments of
the English physicians have placed this truth, as it respects man,
beyond a doubt. It is unnecessary to give the detail of these well
known experiments, in which the mercury was seen to descend in the
thermometer, when the bulb was placed in the mouth and in which the
skin became covered, in a heated room, with the aqueous vapours of the
air, which the greater cold of the body condensed upon its surface. A
slight attention to animals with cold blood, living in warm climates,
proves the same thing. I will make one remarkable observation upon this
point, it is, that most reptiles, whose temperature is much less than
that of the mammalia and of birds, and who consequently are brought
much nearer than them to the temperature of winter, cannot however
support it. They become torpid and sleep in subterraneous places, the
heat of which remains nearly uniform like that of cellars, and do not
awake till the milder temperature of spring stimulates them.