THE SKIN.
617. The skin is a membrane which envelops the muscles and other parts
of the system. In youth, and in females particularly, it is smooth,
soft, and elastic. In middle age, and in males, it is firm and rough
to the touch. In old age, in persons who are emaciated, and about the
flexions of the joints, it is thrown into folds. The interior of the
body, like the exterior, is covered by a skin, which, from the
constantly moistened state of its surface, is called the mucous
membrane. At the various orifices of the body, the exterior skin is
continuous with the internal.
ANATOMY OF THE SKIN.
618. The SKIN, to the naked eye, appears composed of one membrane. But
examination has shown that it consists of two layers of membrane,
namely, the _Cu'ti-cle_, (scarf-skin,) and the _Cu'tis Ve'ra_, (true
skin.) These layers are widely different from each other in structure,
and perform very different offices in the animal economy.
619. The CUTICLE (sometimes called the _ep-i-derm'is_) is the external
layer of the skin. This membrane is thin and semi-transparent, and
resembles a thin shaving of soft, clear horn, and bears the same
relation to other parts of the skin that the rough bark of a tree does
to the liber, or living bark. The cuticle has no perceptible nerves or
blood-vessels; consequently, if it is cut or abraded, no pain will be
felt, and no fluid will ooze from it.
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617. What is the skin? Mention its different appearances in its
different conditions in the human frame. Is the interior of the body,
as well as the exterior, covered by a skin? What is the interior
membrane called? Why has it received this name? 618-636. _Give the
anatomy of the skin._ 618. What is said of the skin? What is said
relative to these layers of membrane? 619. Describe the cuticle. What
name is sometimes applied to the cuticle?
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_Experiment._ Pass a pin through the portion of the cuticle that
skirts the nails, or remove a thin shaving from the palm of the hand,
and no painful sensation will be experienced unless the pin or knife
penetrates deeper than the cuticle.
620. This membrane varies in thickness on different parts of the
body,--from the thin, delicate skin upon the internal flexions of the
joints, to the thickened covering of the soles of the feet. The
greater thickness of the cuticle of the palms of the hands and soles
of the feet, is manifestly the intentional work of the Creator; for it
is perceptible in infants, even at birth, before exercise can have had
any influence.
621. The CUTIS VERA (sometimes called the _co'ri-on_) is composed of
minute fibres, which are collected into small bundles or strands.
These are interwoven with each other so as to constitute a firm,
strong, and flexible web. In the superficial part of the true skin,
the web is so close as to have the appearance of felt-cloth; but more
deeply, the pores become progressively larger, and, upon the lower
surface, have a diameter of about a line, or one twelfth of an inch.
This gives the under surface the appearance of a coarse web. The
strands of the under surface of the true skin are connected with the
fibrous web, in which the sub-cutaneous fat of the body is deposited;
while the upper surface gives support to the sensitive, or papillary
layer, which is bedded upon it.
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Give the experiment. 620. What is said of the thickness of the cuticle
in different parts of the body? 621. Describe the cutis vera. By what
name is it sometimes called? What is the appearance of the upper
surface of the cutis vera? Of the under surface?
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_Observation._ When the skins of animals are immersed in a strong
solution of oak or hemlock bark, a chemical union takes place between
the gelatin, of which the true skin is mostly composed, and the tannin
of the bark. By this process leather is formed, and its peculiar
markings are owing to the papillary layer.
[Illustration: Fig. 111. An ideal representation of the papillae. 1, 1,
The cutis vera. 2, 2, The papillary layer. 3, 3, The arteries of the
papillae. 4, 4, The veins of the papillae. 5, 5, The nerves of the papillae.]
622. The sensitive layer of the skin is thin, soft, uneven, pinkish in
hue, and composed of blood-vessels, which confer its various tints of
red; and of nerves, which give it the faculty of sensation. The
unevenness of this layer is produced by small, elongated, conical
prominences, called _Pa-pil'lae_.
623. Each PAPILLA is composed of a minute artery, vein, and nerve.
Some of the prominences are arranged in concentric ovals, as may be
seen on the ends of the fingers; others are more or less parallel, and
pursue a serpentine course; some suddenly diverge, and again reunite,
as may be seen in the palm of the hand. Papillae are found in every
part of the skin. Consequently, their number is very great.
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How is leather formed? 622. What is the appearance of the sensitive
layer? What causes the unevenness of this layer? Explain fig. 111.
623. Describe the papillae.
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624. The cutis vera contains not only _Arteries_, _Veins_, and
_Nerves_ but _Lymphatics_, _Oil-Glands_ and _Tubes_, and _Perspiratory
Glands_ and _Tubes_.
[Illustration: Fig. 112. The arteries and veins of a section of the skin.
A, A, Arterial branches. B, B, Capillary, or hair-like vessels, in which
the large branches terminate. C, The venous trunk, collecting the blood
from the capillaries.]
625. The ARTERIES AND VEINS of the skin are very numerous. The larger
branches of the arteries pass through the open meshes of the true
skin, and are subdivided into a myriad of minute capillary vessels,
which form a beautiful net-work on the upper surface of the true skin.
This vascular net sends a branch to each of the papillae, which opens
into and terminates in a minute vein. The capillary veins are as
numerous as the arteries which they accompany. They unite and form
larger trunks, as small springs from the hill side coalesce to form
rivulets.
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624. What vessels are found in the cutis vera? Explain fig. 112. 625.
What is said of the cutaneous arteries? Of the cutaneous veins?
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626. The NERVES that are spread over every part of the sensitive layer
of the true skin, proceed from the spinal cord. As a proof of the
great number of nervous filaments in the skin, no part of this tissue
can be punctured with a fine needle without transfixing a nerve, and
inducing pain. In some parts of the system, however, the nerves are
more abundant than in others; where the sense of feeling is most
acute, we find the greatest number of nerves, and those of the largest
size. Those parts that are most exposed to injury are most sensitive.
_Examples._ 1st. The conjunctiva, or skin of the eye, is pained by the
presence of a particle of dust, because it would render vision
imperfect.
2d. The lungs, also, would be injured by the smallest particle of
matter; they are therefore protected by the exquisite sensitiveness of
the lining membrane of the trachea, so that a particle of food or dust
is ejected by a convulsive cough before it reaches the lungs.
627. The nerves are more numerous in the upper than lower extremities;
in greater numbers upon the palm than the back of the hand. They are,
likewise, more abundant and larger at the extremities of the fingers,
and in the lips, than in any other part of the skin.
_Observation._ The proboscis of the elephant, the extremities of the
tails of certain species of monkeys, and the tentacula of some kinds
of fish, receive a more abundant supply of sensitive nerves than other
parts of their systems.
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626. Where do the nerves of the skin proceed from? Are they numerous
in this membrane? How is it proved? What is said of those parts most
exposed to injury? Give example 1st. Example 2d. 627. Mention the
difference in the distribution of the nerves in various parts of the
body. Is this difference found in the lower order of animals?
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628. In the small papillae, the nerve forms a single loop, while in
papillae of larger size, and endowed with a power of more exalted
sensation, the nerve is bent several times upon itself previous to
completing the loop. These little loops spring from a net-work of
nerves, imbedded in the upper porous layer of the true skin, at the
base of the papillae. This net-work of nerves receives its influence
through nerves which take their winding course through the fat
distended openings of the deeper layers of the true skin.
[Illustration: Fig. 113. 1, 1, The cuticle. 2, 2, The colored layer of
the cuticle. 3, 3, The papillary layer, exhibiting the nerves as they
form loops. 4, 4, The net-work of nerves. 5, 5, The true skin. 6, 6, 6,
Three nerves that divide to form the net-work (4, 4.) 7, 7, 7, The
furrows between the papillae. 8, 8, 8, Three papillae magnified fifty
diameters.]
629. The LYMPHATICS are found in great numbers in the true skin, and
they are so minute that they cannot be seen with the naked eye; but
when these hair-like vessels are injected with quicksilver, (a work of
great difficulty,) the surface injected resembles a sheet of silver.
In this way their existence can be imperfectly demonstrated. They are
a part of the vascular net-work situated upon the upper surface of the
true skin. Each papilla is supplied with a lymphatic filament, the
mouth of which opens beneath, and lies in contact with the under
surface of the cuticle. This net-work of vessels communicates through
the open meshes of the true skin with larger lymphatic trunks, that
open into the venous system.
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628. How are the nerves of the small papillae arranged? How in the
large papillae? What does fig. 113 represent? 629. What is said of the
cutaneous lymphatics? How is their existence proved?
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[Illustration: Fig. 114. A plexus of lymphatic vessels in the skin,
considerably magnified from an injected preparation.]
630. The OIL-GLANDS are small bodies imbedded in the true skin. They
connect with the surface of the skin by small tubes, which traverse
the cuticle. In some parts, these glands are wanting; in others, where
their office is most needful, they are abundant, as on the face and
nose, the head, the ears, &c. In some parts, these tubes are spiral;
in others, straight. These glands offer every shade of complexity,
from the simple, straight tube, to a tube divided into numberless
ramifications, and constituting a little rounded tree-like mass, about
the size of a millet seed.
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Of what are they a part? 630. Describe the oil-glands. With what do
they connect? Do they exist in every part of the body? Of what form
are their tubes?
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631. In a few situations, these small glands are worthy of particular
notice, as in the eyelids, where they possess great elegance of
distribution and form, and open by minute pores along the lids; in the
ear-passages, where they produce that amber-colored substance, known
as the _ce-ru'men_, (wax of the ears,) and in the scalp, where they
resemble small clusters of grapes, and open in pairs into the sheath
of the hair, supplying it with a pomatum of Nature's own preparing.
The oil-tubes are sometimes called the _se-ba'ceous fol'li-cles_.
[Illustration: 4. A small hair from the scalp, with its oil-glands. The
glands (A) form a cluster around the shaft of the hair-tube, (C.) These
ducts open into the sheath of the hair, (B.) All the figures, from 1 to
4, are magnified thirty-eight diameters.]
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631. What is said of these tubes in the eyelids? In the ear? In the
scalp? What are these glands sometimes called?
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_Observation._ Among the inhabitants of cities, and especially in
persons who have a torpid state of the skin, the contents of the
oil-tubes become too dense and dry to escape in the usual manner. Thus
it collects, distends the tube, and remains until removed by art. When
this impacted matter reaches the surface, dust and smoke mix with it,
then it is recognized by small, round, dark spots. These are seen on
the forehead, nose, and other parts of the face. When this matter is
pressed out, the tube gives it a cylindrical form. The parts around
the distended tubes sometimes inflame. This constitutes the disease
called, _"ac'ne punc-ta'ta."_
632. The PERSPIRATORY APPARATUS consists of minute cylindrical tubes,
which pass inward through the cuticle, and terminate in the deeper
meshes of the cutis vera. In their course, each little tube forms a
beautiful spiral coil; and, on arriving at its destination, coils upon
itself in such a way as to constitute an oval-shaped, or globular
ball, called the _perspiratory gland_.
633. The opening of the perspiratory tube on the surface of the
cuticle, namely, "the pores," is also deserving of attention. In
consequence of its extremity being a section of a spirally-twisted
tube, the aperture is oblique in direction, and possesses all the
advantages of a valvular opening, preventing the ingress of foreign
injurious substances to the interior of the tube and gland.
634. "To arrive at something like an estimate of the value of the
perspiratory system, in relation to the rest of the organism, I
counted the perspiratory pores on the palm of the hand, and found 3528
in a square inch. Now each of these pores being the aperture of a
little tube about a quarter of an inch long, it follows, that in a
square inch of skin on the palm of the hand there exists a length of
tube equal to 882 inches, or 73 1/2 feet. Surely such an amount of
drainage as seventy-three feet in every square inch of skin--assuming
this to be the average for the whole body--is something wonderful and
the thought naturally intrudes itself, What if this _drainage_ be
obstructed?
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What is said of the retention of the unctuous matter in the oil-tubes?
632. Of what does the perspiratory apparatus consist? 633. What is
peculiar in the opening of the perspiratory tubes on the surface of
the cuticle? 634. How many perspiratory pores did Dr. Wilson count
upon a square inch of skin on the palm of the hand?
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[Illustration: Fig. 116. A perspiratory gland from the palm of the hand,
magnified forty diameters. 1, 1, A twisted tube composing the gland. 2,
2, The two excretory ducts from the gland. These unite to form one spiral
tube, that perforates the cuticle, (3,) and opens obliquely on its
surface at 4. The gland is imbedded in cells filled with fat, which are
seen at 5, 5.]
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What does fig. 116 represent?
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635. "Could we need a stronger argument for enforcing the necessity of
attention to the skin? On the pulps of the fingers, where the ridges
of the sensitive layer of the true skin are somewhat finer than in the
palm of the hand, the number of pores on a square inch a little
exceeded that of the palm; and on the heels, where the ridges are
coarser, the number of pores on the square inch was 2268, and the
length of the tube 567 inches, 47 1/4 feet.
636. "To obtain an estimate of the length of tube of the perspiratory
system of the whole surface of the body, I think that 2800 might be
taken as a fair average of the number of pores in the square inch; and
consequently, 700, the number of inches in length. _Now, the number of
square inches of surface in a man of ordinary height and bulk is 2500;
the number of pores, therefore, 7,000,000; and the number of inches of
perspiratory tube is 1,750,000; that is, 145,833 feet, or 48,611
yards, or nearly TWENTY-EIGHT miles!_"--_Wilson._
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Give other computations in this paragraph. 635. What is said of the
number of these pores on the pulp of the fingers? On the heels? 536.
What is an average number of pores and length of tube of the whole
surface of the body? Give the summary of the number of pores, and
number or inches of perspiratory tube.
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