cannot say what is the nature of these membranes. But authors in
general agree in attributing to them a soft and pulpy nature, which
does not appear to me to accord with that of the epidermis. I have
many times seen at the Hôtel Dieu white membranes detached from the
œsophagus after poisoning with the nitric acid. But these membranes
are evidently the superficial portion of the mucous organ, which is
disorganized, and thrown off by suppuration which takes place below. It
is thus that cutaneous eschars fall off in the form of membranes from
large burns; in this way the osseous layers are formed in necrosis,
which are only the superficies of the bone that dies and is detached in
a lamellated form.
From this, the existence of the epidermis of the deep seated mucous
surfaces appears to me to be very uncertain, and cannot be admitted
till a new examination, which will, I think, prove rather against
than in favour of its existence. What is the place in which the
epidermis terminates that lines the origin of the mucous surfaces,
or if it exists everywhere, where does it begin to become no longer
apparent from the action of our different reagents? We cannot, I think,
determine with precision; it diminishes in an insensible manner, and is
lost as it were by degrees.
ARTICLE THIRD.
OF THE NAILS.
All the fingers have at their extremity, on the outer side or that of
extension, hard, transparent and elastic layers, of the nature of the
horns of many animals, and which are called nails.
I. _Forms, Extent, Relations, &c._
The nails of man differ from those of most other animals, in their
breadth and want of thickness. The first makes them better adapted to
support the extremity of the fingers, which is broader than in most
animals for the perfection of touch; the second renders them less fit
to serve for defence or as a means of aggression.
Most people cut their nails even with their fingers, so that the length
of these bodies which is seen is not what is natural to them. When
allowed to grow, they lengthen and turn over on the side of flexion,
and cover entirely the lower end of the fingers. This growth has a
certain limit which the nail cannot pass, and which it attains when it
exhibits at its extremity a cutting and sharp edge. As long as this
edge has the appearance of having a part cut off, the nail continues to
grow.
We usually think that the habit of cutting our nails is a thing of
mere decorum. But if we reflect a little upon society and the numerous
arts to which it gives rise, upon the perfection, delicacy, precision
and rapidity of the motions which the fingers are often forced to
execute, upon the necessity of approximating them, crossing them in
a thousand different ways, &c. we shall soon see that this habit is
almost inevitably the result of the social state, and what appears to
us the effect of fashion is that of necessity. The sense of touch in
man in a natural state is coarse and obscure; it is only necessary that
he should seize objects destined for his nourishment, his defence, his
aggressions, &c. that he should climb especially and attach himself to
trees to keep himself upon them; now his nails are for this purpose
of great use. What he loses in this respect in society, he seems to
gain by the precision and extent which are added to his touch, and
by the faculty which the fingers acquire of distinguishing the most
delicate tangible qualities. In the first state, his hands are of great
assistance to him in locomotion. In the second, they contribute hardly
at all to this use, and they gain in the partial motions of their
fingers what they lose in their motions as a whole, which become of
less urgent necessity.
The nail has three distinct parts in the natural state; one posterior,
concealed on both sides by the integuments; another middle, free only
on one side, and the third anterior, without adhesion at either side.
The posterior portion of the nail is nearly a sixth part of its extent.
Its convex surface adheres very intimately to the epidermis, which
goes in the following manner to fix it. After having covered over the
portion of the finger corresponding with flexion, it is reflected
upon the concave edge where the skin terminates and where the nail
begins to become external; it commonly forms all around this edge a
kind of small string that is very distinct and has a small groove in
the top of it, and which is evidently composed entirely of epidermis,
since we can cut the whole of it without giving any pain, and which is
afterwards easily reproduced. After having formed this string, which
is in the form of a parabola, the epidermis is again reflected, passes
between the skin and the nail, and is glued, if we may so say, to the
concave surface of the latter, without being intermixed with it; for
we can remove it with ease by scraping with a scalpel. So that the
dermis which covers the superior portion is really between two layers
of epidermis. After having thus fixed the nail, and having arrived at
its posterior edge, the epidermis is continued and identified as it
were with this edge, whose evident delicacy and softness approximate it
in nature to this membranous layer. Hence it follows that without the
adhesion of the epidermis to the nail, there would be between them, a
kind of cul-de-sac. Some authors have thought that the extensor tendon
is extended as far; but it is easy to see that it does not go beyond
the tubercle which terminates behind the phalanx. The nail does not
reach this tubercle, there is a space of three lines between them. The
concave surface of the posterior portion of the nail corresponds with
the same substance as the middle portion.
This middle portion is bare on its convex surface, which is smooth,
whitish behind where this colour forms a kind of half moon, reddish in
the greater part of its extent, a colour which is foreign to it and
which it derives from the subjacent texture. Upon the sides, the skin
covers this surface a little, and terminates afterwards by continuing
the concave and free edge of which we have spoken. The epidermis forms
also in this place a small string which is continued on each side with
that pointed out above; then it unites to the nail and adheres to its
lateral edges with which it is identified. The concave surface of
this middle portion is fixed in front by the epidermis, which, after
having covered the extremity of the fingers, and having arrived at the
place where the nail ceases to be free, is separated from the dermis,
and adheres to the whole length of the nail in a curved line; then by
mixing with it, it seems to form its internal lamina. The dermis on the
contrary is continued on the convexity of the last phalanx, has there a
remarkable consistence, a reddish appearance, and a texture like pulp
and wholly different from what is observed elsewhere; more vessels run
through it; there is no distinct space in it, and no elongation goes
from it to the surface of the nail of which the epidermis forms a part.
We do not see on this surface, as on those of the other parts of the
epidermis, those threads, which are the remains of the broken exhalants
and of which we have spoken; thus the sweat never passes through the
nail. There is neither any oily oozing upon its surface; whence it
follows that water is not formed into little drops on the exterior of
its horny laminæ. Hence the nail is evidently insulated from all the
other organs except the epidermis, with which it is continued on its
concave face and especially on its posterior and lateral edges. Thus
observe that when collections of pus or other affections have broken
this continuity behind or on the sides, the whole of the nail, though
unaffected in the middle falls off.
The free or anterior portion of the nail is of a length which it is
difficult to determine. I have never seen it allowed to take its
natural growth. I have only observed that if it is permitted to grow to
a considerable size, we see evidently that it has a greater thickness
than the posterior and middle portion. In general, the thickness,
resistance and hardness of the nail increase in a gradual manner from
the posterior to the anterior part; we shall now examine to what this
is owing.
II. _Organization; Properties, &c._
In order to observe the organization of the nails advantageously, it is
necessary to take those that are very distinct, as those of the great
toe, the thumb, &c. We then see evidently that a single lamina occupies
the whole of their convex surface. Behind, this lamina exists alone;
hence the extreme thickness of the nails at this place. But as we
examine towards the front, we see new laminæ successively added to it,
on the concave surface; so that the nail becomes successively thicker.
These laminæ can be easily raised up layer by layer. The most anterior
are the shortest. They often exhibit upon the concave surface of the
nail an infinite number of very evident small striæ, all longitudinal
and parallel, and which make us attribute to it a fibrous texture. At
other times this arrangement is less evident.
What is the nature of the laminæ which form the nails? I believe that
they are almost precisely the same as the epidermis. What proves it
is, 1st, that the most superficial is evidently continued with it by
its edges; there is no intermediate agent between them. 2d. I have
already observed that the nails are detached, and then regenerated
exactly like the epidermis. They have two modes of increase; one in
length, when we cut their extremities; the other in thickness, when we
detach only a lamina, which is soon formed again. When the whole of the
nail comes off, all the portion of the dermis which covers the back of
the last phalanx, contributes at the same time to form it anew by its
external surface. 3d. There is the same obscurity in the vitality of
the nails as in that of the epidermis. No trace of animal sensibility
is discoverable in them. The excruciating pain that is experienced
when they are pulled out arises solely from the sensibility of the
subjacent pulpy texture; it is from the same cause as in pulling out
the hair. There is no organic sensibility, no internal circulation and
consequently no heat inherent in the texture of the nails; thus the
horns of animals are nearly of the same degree of temperature as the
atmosphere, whilst some external productions with evident vital forces,
though raised up like horns, have a temperature equal to that of the
body. Such are the combs of the cock of our country, and those which
are more striking of the cock-turkey. Compare these excrescences with
those on the feet of these animals, which are horny, and the difference
of temperature is evident. 4th. The nails give out when burning a
disagreeable odour, analogous to that of the epidermis under the same
circumstances; they exhibit then the same phenomena. Their combustion
is supported, like that of the epidermis, by an oil of which they
contain a great quantity. 5th. If maceration and stewing do not produce
upon the nails that want of consistence, that kind of brittleness,
if I may so express myself, which they produce upon the epidermis, it
appears to be owing only to their greater solidity. 6th. The action of
the nitric, sulphuric acids, &c. has exhibited to me nearly the same
phenomena as upon the epidermis.
Every thing then appears to establish the most exact analogy in
the composition, organization and properties of the nails and the
epidermis. There is no doubt a difference of principles between them,
since the appearance is not the same, and since, though many epidermoid
layers may be in juxta position as on the soles of the feet and the
palms of the hands, they do not exhibit the form and texture of the
nails; so that we cannot consider these as mere layers of epidermis
applied to each other. Chemists must ascertain what these differences
are, which are certainly very slight. Thus nature often employs
indifferently the two organs for the same uses; it is thus that on the
sole of the foot of man and many analogous species, there is a thick
epidermis; whilst on the feet of animals with hoofs, we see a horny
substance of the nature of the human nail.
An evident proof of the slight degree of internal motion which is
going on in the epidermis and the nails, of the kind of inertia in
which they are in relation to the constant motion of composition and
decomposition, which constitutes nutrition, and of the insensibility
which they exhibit to various excitants, is the ease with which they
are penetrated by different colouring substances, and the very long
time they retain them. We know this effect with regard to the nails
of dyers. Many savage people who paint the face, various parts of the
body, and often even the whole of the external surface of the body,
preserve for a long time, without a new coat, the colour which they
have artificially given themselves. I have removed the epidermis of
a portion of skin of the arm of a dead body, which was coloured blue
during life; this colour was not only on the surface of the membrane,
but penetrated the whole of it, like a piece of cloth that had been
soaked in it. Yet the pores were as evident as before, and the sweat
could pass through them; I presume this secretion goes on as usual in
savages who paint the skin. Thus the cloth which is immersed in a dye,
has not its pores closed by it. I may make use of this comparison, as
the epidermis and the nails are really species of inorganic bodies. Lay
any organ bare and paint it in this way; the colour, together with the
contact of the air will irritate and inflame it, and the suppuration
arising from this inflammation will soon throw out the colouring
particles, which nutrition would have done, if inflammation had not.
There is a means however which can perpetuate the colour, even upon
organs, which, very sensible like the skin, are constantly subject to
the double nutritive motion; it is that of using the colours with a red
hot iron. It is in this way I am convinced that the letters or coloured
figures which most soldiers mark upon themselves, with a red hot pin,
have their seat not only in the epidermis, but also in the chorion
itself.
_Development._
The nails have in the fœtus a very considerable consistence, whilst
the skin is still pulpy; but their tenuity is then extreme. But they
thicken and acquire greater consistence as the fœtus increases in
size. They have not at birth a length proportionable to what they are
afterwards to have. They do not extend beyond the ends of the fingers
which are often much the longest; so that it is not till after birth
that they are bent over and exceed the fingers in length, for both of
these would be useless in the womb of the mother, as there is nothing
there for the fœtus to seize upon. Their transparency allows us
evidently to see, at the moment of birth, first the black colour of the
blood which before circulated in the arteries, and then the vermilion
colour which respiration suddenly imparts to it. As we advance in age,
the nails grow in the same proportions as the epidermis, but they have
nothing peculiar in their growth. In old people they become extremely
thick.
These organs experience during life those diseases only which
are analogous to those of the epidermis. These are excrescences,
augmentations of size, &c. and other productions, the texture of
which is precisely the same as that of the nail, and in which there
is neither more sensibility, nor more circulation, nor more heat, nor
more life; a remarkable character which distinguishes them from those
tumours which arise upon the other organs with very active vitality, as
upon the skin, the muscles, &c. tumours the texture of which is very
different from that of the organs which have produced them, and which
most usually have properties entirely different. But the epidermoid
excrescences are in every respect analogous to the epidermis.
PILOUS SYSTEM.
The adjective by which I characterize this system, is derived from the
latin substantive which signifies the organs of which it is composed.
Hair is found less generally upon man than upon most other animals.
It forms upon them a kind of covering external to the skin, which,
lessening in part the contact of external bodies, makes the cutaneous
animal sensibility perform a less important part, and establishes
less numerous relations between these bodies and them. External life
is then, in this respect, more limited in them than in man, in whom a
delicate epidermis and a few hairs thinly scattered over it, separate
the organ of feeling from surrounding objects, the least impression of
which is felt, and which, owing to this, keep the animal sensibility
in permanent activity; thus man is designed to live more without than
within himself. The pleasures of reproduction and digestion constitute
exclusively the happiness of animals. That of man is in part the
result of them; but an order of pleasures wholly different, purely
intellectual and in relation only with external sensations, enlarges
immensely by its presence, and contracts by its absence, the field of
this happiness.
The hair of man covers especially the cranium, some parts of the face,
the front of the trunk, the genital organs, the extremities, &c. The
quantity varies remarkably, as well as the form, length, &c. In order
to form an accurate idea of it, we shall now consider it separately in
the different organs, we shall then treat of its general organization,
properties and development.
ARTICLE FIRST.
EXAMINATION OF THE PILOUS SYSTEM IN THE DIFFERENT REGIONS.
This system must be considered on the head, the trunk and the
extremities.
I. _Pilous System of the Head._
The head is the part of the body in which this system predominates;
it covers the whole cranium and defends it against the impression of
external bodies, as the hairy coat of quadrupeds defends them. Thus
this part is the least capable of exercising the sense of touch, either
from the obscurity of the animal sensibility arising from this hairy
covering, or because its convex form allows it to be in contact with
external bodies only by a small surface.
The face is less generally covered with hairs, though many are found
upon it, especially in men. This part, in which in a very small space
are collected the greatest number of our means of communication with
external objects, viz. the organs of taste, smell, sight and even
hearing, has but very little to do with the sense of touch, on account
of its villous arrangement. Its form is also badly adapted to this
sense. The mouth which is flattened cannot be applied to external
bodies. Thus whilst the snout which is elongated in most quadrupeds,
performs the double function of first feeling all bodies, turning them
in various directions in order to ascertain their tangible qualities,
and then of seizing them for nourishment, the mouth of man serves only
for this last use; it is the hands which are destined for the first.
Thus observe that all animals, even the most of those with clavicles,
almost uniformly direct their snout towards the earth, whilst the mouth
of man is naturally destined to an opposite position.
_Of the Hairs of the Head._
They occupy upon the cranium all the space which corresponds with
the occipital, parietal, the squamous portion of the temporal and a
small portion of the frontal bones. Their limits do not vary on the
sides; they always correspond above the ear. Behind, they sometimes
go down upon the superior part of the neck; at others, they do not
extend beyond the head. In applying blisters on the ligamentum nuchæ,
we observe in this respect, almost as many varieties as there are
subjects. We know how variable these limits are in front. Sometimes
extended lower down, sometimes carried higher up, sometimes describing
a curved line, and at others forming a real triangle the anterior point
of which corresponds with the median line, they have really nothing
constant.
These inequalities alone determine the breadth or narrowness of the
forehead, whilst its degrees of inclination depend solely upon the bone
which forms it. It is in this way that the hair contributes a little
to the expression of the face; I say a little, for it is less to the
breadth of the forehead than to its approximation to a perpendicular,
that we attach the ideas of majesty and greatness which characterize
heroes and gods. The poets, as we know, have particularly celebrated
the forehead of the god of thunder. Observe in relation to this subject
that there is a great difference between that which expresses majesty
or abjectness in the face, from that which serves there to express the
passions. It is the osseous structure of this region and the degree
of inclination resulting from this structure, which serve for the
first use, and it is especially the muscular motions which contribute
to the second. Why? Because majesty, grandeur, &c. are especially
connected with the extent of the understanding, and the understanding
has its seat in the brain, and because the various capacities of the
cranium, which contain this organ, and which correspond with its
various degrees of development, have inevitably an influence upon the
different dimensions of the face. Now as the bony structure is a thing
constant and invariable, the air of majesty or abjectness remains
always imprinted upon the face. On the contrary, the passions which
especially affect the epigastric organs, which afterwards excite the
facial muscles, have necessarily a transitory expression.
The number of hairs is very variable on the same surface. In some
people they are very close together and even all touch; in others more
thinly scattered, they allow in part the skin of the cranium to be seen
in their interstices, a circumstance which is either owing to original
conformation, or to a disease which makes them fall out in part. They
have, like the nails, a determinate growth which they do not exceed.
We know but little of the limit of this growth. Yet we have seen them
reach to the waist, the thighs, and the legs even. It appears that
in women they have a greater growth; we might say, that nature has
compensated this sex in this way for the want of hair in many other
parts. Floating upon the shoulders, the breast, the trunk, &c. they
form in the natural state a sort of protection from the injuries of
the air and the light. Their extent evidently proves that man was
destined to an erect attitude. In fact, in the attitude of quadrupeds,
they would trail much upon the earth, and form an obstacle to motion.
The hair of no animal, I believe, in a natural attitude retards his
progress so much, as the hair of man then would.
Man, who opposes nature in every thing, has made it a habit in most
societies to cut the hair, the beard, &c. By common people, it is
considered merely a thing of fashion; by the physician, as a practice
which has perhaps a greater influence than is thought upon the
functions. In fact, in the natural state when the pilous system has
once acquired its growth, it no longer exhibits the constant motion
of composition and decomposition. On the contrary, in man who cuts
it, it is constantly the seat of this motion and of that of growth.
This practice perpetuates then the phenomena which take place in them
in infancy, and consequently keeps up there a more active work, which
perhaps is performed at the expense of that of many other parts.
The natural difference of the hair has much influence upon its length;
that which is smooth and curls but little is in general the longest.
The more it has the opposite characters, the shorter it is, as is
proved by that of negroes and those white people whose hair curls like
theirs.
The tenuity of the hair is very great, yet its resistance is in
proportion very considerable. There is no part in the economy, not
even those of the fibrous system, which can support so great a weight
in proportion to its size. Thus woven strings of hair would have an
enormous resistance, if they were sufficiently long to be employed for
different uses.
The colour of the hair varies remarkably according to country,
latitude, climate, temperature, &c. This colour is even, like that
of the skin, a characteristic attribute of the different human races.
Naturalists have been much occupied with this subject, and I refer to
their works.