as the fibrous substance is constantly tending to a predominance over
the gelatinous. Less blood seems to be carried to it. It becomes less
and less disposed to eruptions, so common in youth and infancy, &c. I
will not speak of its other differences; for all that we have said of
it in the preceding articles relates especially to the adult age.
I will only observe that if, during the greatest part of life, the skin
be so fruitful a source of diseases, and the various alterations it
experiences produce so frequent disorders in the internal organs, it
is only owing to the varied causes of excitement to which it is every
instant subjected. If the glands, the serous surfaces, &c. have an
influence less frequently upon the other organs, it is because being
deeply situated, and almost always in contact with the same excitants,
they are not subject to so many revolutions in their vital forces. The
secreted fluids and those exhaled in the serous and synovial systems
are not, for the same reason, so much subject to those considerable
increases, and those sudden suppressions which so frequently happen to
the sweat.
Observe that society has also multiplied to a great extent the
injurious excitements to which the skin is subjected. These excitements
consist especially in the rapid passage from heat to cold, which
makes the latter act very powerfully upon the cutaneous sensibility,
which like that of all the other systems, answers so much the more
to excitements made upon it, as they are different from those, whose
action they had previously experienced. In the natural state, there
is only the succession of the seasons; nature knows how to connect
insensibly heat with cold, and to make the transition but rarely
abrupt. But in society, the different garments, the artificial degrees
of temperature of our apartments, degrees differing at first from that
of the atmosphere, then varying greatly from each other, so that the
same man who in winter enters thirty apartments, is often subjected to
thirty different temperatures; the hard labour in which most men are
engaged, and which makes them sweat copiously, every thing incessantly
presents numerous causes which make the vital forces of the dermoid
system vary rapidly. Thus the bronchial mucous surface is constantly
in contact in cities, with a thousand excitements that are continually
renewed, and with which the air is not charged in a natural state. Thus
the alimentary substances, continually varying in their composition,
temperature, &c. change the excitement of the gastric mucous surface,
and are the source of many affections, from which most animals are
exempt by the uniformity of their food.
If the skin and the mucous surfaces were always kept at the same
degree of excitement by the constant uniformity of the stimuli, they
would certainly be a much less fruitful source of diseases, as is
clearly proved by the fœtus, which is hardly ever sick, because all the
external causes which act upon its mucous and cutaneous sensibility,
as the heat, the waters of the amnios and the parietes of the womb,
do not vary until birth. At this period, animals brought into a new
medium, find many more varieties in the stimuli which act upon them,
even in a natural state and far from society; thus their diseases are
naturally much more frequent after than before birth. In society, in
which man has increased four, six and even ten times the number of
the stimuli which affect the surfaces destined to be in contact with
the external bodies, is it astonishing that the diseases should be so
disproportioned to those of animals?
IV. _State of the Dermoid System in Old Age._
Towards the decline of life, the dermoid system becomes more and more
firm and compact; it is softened with great difficulty by ebullition.
The gelatine, which it yields, is less abundant and more hard and
consistent. I think it would not be fit to make any kind of glue, even
the strongest, unless mixed with that of adult animals. Its yellowish
tinge becomes very deep. When it is cooled, it requires a much stronger
and more durable fire to melt it; the fibrous portion of the dermis
which does not melt or at least resists for a long time, is infinitely
greater in proportion. It is like the bones in which the gelatinous
portion is in an inverse ratio, and the earthy portion in a direct
ratio to the age.
The dermoid texture becomes then like all the others, dense and stiff;
it is not proper for our food, the teeth cannot tear it. Prepared with
tannin, it is more resisting and less pliable, and cannot on that
account serve for the same purposes as that taken from young animals.
Every one knows the difference of the leather of calves and oxen,
especially when the latter are old. This difference is owing first to
the thickness, which being much greater in the second than the first,
does not allow it to be so easily bent in different directions; and
then to the nature of the texture itself. Cut in two horizontally a
piece of the leather of an ox; each half will be as thin as a piece
of calves skin, and yet it will be less pliable. I do not estimate
here the varieties which may depend on the greater or less quantity of
tannin that may be combined with it; I suppose the proportions to be
all equal.
Submitted to desiccation, the human dermoid texture becomes much more
stiff in old age than in the preceding ones. Maceration softens it with
more difficulty. The hair of a child falls out much sooner by it than
that of an old person; thus it requires longer to clean the skins of
old animals than those of young ones; tanners know this very well. I
would remark upon this subject, that the skins of animals, having more
hairs pass through them, exhibit in comparison with that of man, an
innumerable quantity of little pores on their external surface; which
favours in them on this surface the action of tannin, which insinuating
itself into the dermoid spaces and filling them completely with a new
substance formed by the combination of tannin with gelatine, occupies
entirely the texture of the spaces. The previous maceration to which
the skin has been exposed, favours not only the removal of the hairs,
but facilitates also to a great degree the entrance of the tannin,
by separating the fibres of the spaces, by making them larger, and
increasing the size of the external pores.
The more we advance in age, the less is the quantity of blood that
penetrates the skin. The redness of the cheeks disappears in old
people. We no longer see then the rosy complexion of the young man and
even of the adult, and which arose from the vessels winding through the
cellular texture of the spaces of the chorion.
The continual pressure of external objects increases then remarkably
the adhesion of the subjacent cellular texture to the dermis. They are
separated from each other with great difficulty by carrying the edge
of a scalpel over the internal face of the chorion; a circumstance
which is owing also to this, that the cellular texture having become
more dense, is less easily torn; for this tearing is then necessary,
considering the continuity of the sub-dermoid layer with that which
enters the spaces. The exterior of the skin is uneven and rough. All
the wrinkles of which we have spoken become infinitely more evident;
many belong exclusively to this age.
The vital forces of the dermoid system are more weakened in old age
than those of most of the others, because it is more excited during
life by external bodies. Most of these bodies then make no impression
upon it. The habit of feeling has blunted the animal sensibility. The
touch is exercised but rarely; for, as I have observed, this sense
requires to put it in action, the previous exercise of the will. We
touch, because we have previously seen, heard, tasted, &c. in order
to correct or confirm our other sensations; now the old man, to whom
every thing around is known, to whom nothing is new, is induced to
touch nothing. Compare in this respect the two extreme ages of life.
The infant, to whom every thing that strikes his eyes, his ears, his
nostrils, &c. is unknown, who finds in every thing that surrounds him
new objects of sensation, wishes to touch and lay hold of every thing.
Its little hands are in continual agitation. To touch is a pleasure to
him, for every new object of sensation is agreeable. If in his last
years, man was transported into the midst of objects that never before
struck his senses, he would oftener exercise his touch; but none of
those things excite him among which he has always lived. Hence why old
age is not the age of enjoyments. In fact all our pleasures are almost
relative; we have but little that is absolute; now as habit blunts all
the relative pleasures, which cease because they have existed, the more
the sensations are accumulated by time, the less there are of new ones
left to be experienced, and the more are the sources of happiness dried
up. For a contrary reason, the happiest age is infancy, because it has
before it the whole field of sensations to go over. Man at every step
of his career leaves behind him a cause of his enjoyments. When arrived
at the end he finds only indifference, a state very suitable to his
situation, since it diminishes the distance that separates life from
death.
The organic sensibility of the skin is not less blunted in old age,
than its animal sensibility; hence the following phenomena; 1st,
contagious miasmata are absorbed with difficulty at this age; almost
all pass over the cutaneous surface with impunity. 2d. The exhalation
of sweat is uniformly less; it is hardly ever subject to those great
increases, that are seen in the adult. 3d. The oily fluid is also
furnished in much less quantity; hence the constant dryness of the
exterior of the skin, the cracking of the epidermis in some cases,
&c. 4th. All the diseases which suppose an increase of this organic
sensibility are much more rare. Erysipelas and the different kinds
of eruptions are a proof of it. When these affections take place,
they have a character of remarkable slowness. 5th. The skin resists
external cold much less; it loses easily the caloric of the body,
which always tends to escape in order to be in equilibrium with that
of the surrounding medium; thus old people are always fond of heat.
6th. I am well persuaded that the skin would resist also less, at this
age, a degree of temperature greater than that of the body, and as it
permits the internal caloric to be easily lost in a colder medium, it
would allow the external to penetrate in a warmer one. It would be very
curious to repeat, on the two extreme ages of life, the experiments of
the English physicians.
EPIDERMOID SYSTEM.
If we examine attentively, it is easy to perceive the immense
difference there is between the preceding system and this, which
physiologists have considered as one of its dependancies. Organization,
properties, composition, functions, growth, &c. every thing differs in
the two. By explaining these, the line of demarcation that separates
them will be perceived.
I rank in this system, 1st, the external epidermis; 2d, that which is
spread upon the mucous system, or at least upon one of its parts; 3d,
the nails. Though these last differ very much from the epidermis in
their external appearance, yet they resemble it in so many respects,
that it is difficult not to place them in the same system. In fact the
nails serve as an epidermis for the skin which is subjacent to them;
they are continued with that of the fingers in an evident manner, are
detached and regenerated during life with the same phenomena. The
composition appears to be very analogous. The kind of excrescences is
the same. After death, the nails are detached by the same means as the
epidermis, and then make, as it were, a part of it.
ARTICLE FIRST.
OF THE EXTERNAL EPIDERMIS.
The external epidermis is a transparent membrane, more or less thick,
according to the regions in which it is examined, covering everywhere
the skin, and receiving immediately the excitement of external bodies
which would act too powerfully upon this.
I. _Forms, Relations with the Dermis, &c._
We see upon the epidermis the same wrinkles as upon the skin, because
being exactly contiguous, both wrinkle at the same time. Different
pores open on its surface after having passed through it. Some transmit
the hairs; these are the most apparent; others give passage to the
exhalants. We do not see these in the natural state, because their
course is oblique, and they open between two small layers, which,
being in contact with each other when we do not sweat, conceal their
termination. But if, the skin being very dry, we suddenly sweat, as
after drinking tea, then the little drops which escape from the whole
cutaneous surface, not having had time to run together, but remaining
separate, we see, by the places where they are, the orifice of the
exhalants. Besides, if we examine against the light a considerable
portion of epidermis, its transparency allows us to see many small
pores separated from each other by interstices, and which pass through
it in an oblique direction. It is only in the soles of the feet and the
palms of the hands that we cannot make this observation, which is owing
to the thickness in those parts. It is impossible to distinguish in
these pores the absorbent orifices from those of the exhalants, even
when mercury enters the first by friction.
The internal surface of the epidermis adheres very closely to the skin.
The means of union are at first the exhalants, the absorbents and the
hairs, which in passing through the first, adhere to it more or less,
and thus fix it to the second, from which they arise. By separating
the epidermis by maceration, which is the most proper means, we see on
its internal surface many small elongations of greater or less length,
and which, when examined attentively, appear to be nothing but the
broken extremities of exhalants and absorbents. In fact these little
elongations which are easily raised up, and which then appear like
small ends of thread when they are of some size, but which exhibit only
inequalities when they are left very short, have all of them an oblique
course, and terminate in the pores which, we have said, pass through
the epidermis to go to its surface. Their existence is sufficient, at
the first inspection and without the aid of a microscope, to enable us
to distinguish the internal from the external face of this membrane.
The spaces that separate them are more or less large. About these
spaces, the adhesions are less. It is at this place that the small
epidermoid vesicles are formed with which the skin is covered when
plunged into boiling water. The depressed interstices, which separate
these vesicles, are the places where the exhalants are and which have
prevented the epidermis from being raised up. When ebullition is long
continued, they are detached also.
We cannot doubt then that all these vascular elongations serve
powerfully to unite the epidermis to the chorion. How is the adhesion
formed in their interstices? I know not; but it exists, though it is
less evident. The cellular texture, as I have said, appears to take no
part in it.
Every one knows that many causes destroy the adhesions of the
epidermis, and raise it up. These causes are, 1st, every severe
inflammation, whatever may be its species. We know that after
erysipelas, phlegmon, biles, and cutaneous eruptions of different
natures, the epidermis is always detached; there is then no fluid
that raises it up. The exhalants cannot furnish it, as they are full
of blood; it is dry when detached. 2d. Various cutaneous eruptions,
which have not an inflammatory character, as herpes, &c. also detach
the epidermis at the place where they are. It most commonly comes off
then in the form of dry scales; hence no doubt the idea of some authors
who have attributed to it a scaly structure, which neither observation
nor experiment upon the epidermis in the natural state have proved.
This pealing off in scales is owing to the same cause precisely as the
formation of vesicles which take place the instant after the skin has
been plunged into boiling water, viz. to the greater adhesion of the
exhalant vessels which go to the epidermoid pores. Observe in fact
that it is always in the space between these pores that the scales are
produced, which do not exist in nature, but which arise only from the
manner in which the membrane is raised up. For example, when herpetic
eruptions take place on the chin, the pores through which the hairs
pass are not detached; it is only the epidermis in the space between
these pores; now as these are very near together, these scales are
extremely small; they are almost like dust. 3d. Whenever the epidermis
is raised up by cutaneous inequalities, the least friction detaches it
from these inequalities. Hence why, after strong dry frictions, a rough
skin becomes scaly, whilst a smooth one experiences no alteration; it
is this even, which in the external appearance, contributes much to the
ugliness of the one and the beauty of the other. 4th. After idiopathic
fevers, and even many affections of the internal viscera, the skin
which has felt the sympathetic influence of the disease, becomes the
seat of an alteration which without having any external sign, is
sufficient to break the union of it with the epidermis, which is
everywhere raised up. 5th. We know that the action of a blister, which
draws a large quantity of serum to the external surface of the chorion,
breaks off the exhalants which go from it to the epidermis; so that
this serum is effused under it and forms a more or less considerable
sac. The water does not escape through the open pores, because their
oblique course through the epidermis makes their parietes, when
brought in contact with each other by the pressure of the water,
form an obstacle to it. It is for the same reason, that though these
pores may be very evident, as I have said, in a separate portion of
epidermis when examined against the light, this portion will support
mercury, without giving passage to its particles. 6th. Most of the
preceding means, which produce their effect only by an alteration of
the vital forces, have no effect in raising the epidermis in the dead
body. Putrefaction, maceration and ebullition are those by which it is
effected. All act by breaking the elongations which extend from the
dermis to the epidermis, though the mechanism of this rupture is not
exactly known.
II. _Organization, Composition, &c._
Authors have made many conjectures upon the structure of the epidermis,
which it would be useless to relate here. I shall only speak of what
accurate observation demonstrates. Its thickness is in general very
uniform in all the parts. It has not appeared to me to be increased or
diminished, according to the varieties of thickness of the skin on the
back, the abdomen, the extremities, &c. It is only on the soles of the
feet, the palms of the hands and the corresponding face of the fingers,
that this thickness becomes greater. It is even so great in these
places, that there is no proportion between them and the other parts of
the body as it respects this membrane; it is especially towards the
heel that it exhibits this character. This excessive thickness appears
to be owing to different layers which are applied upon each other, and
which seem to be superadded to the layer of the ordinary epidermis;
but there is also a real difference, though but little known, in the
organization; for example, when the epidermis has been removed from
these parts by maceration, we cannot see, as in the others, those small
appendices or inequalities regularly scattered over it, and which are
the remains of the broken exhalants. In these places these vessels are
torn smoother on the internal surface of the epidermis, on which are
seen only the traces of the wrinkles of which we have spoken.
I attribute to this excessive thickness of the epidermis of the soles
of the feet and the palms of the hands, the difficulty and oftentimes
impossibility of making blisters act in these places, on which I have
often applied them there, because I thought that the sensibility being
greater, they would produce more effect in some diseases. The failure
of my attempts has compelled me to renounce them.
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.