The skin is composed of two principal layers; a thin outer layer, called
the epidermis, cuticle, or scarf skin, and a deeper structure, the true
skin, or dermis. We will describe the latter first.
=The True Skin.=—This structure covers the entire surface of the body.
It varies in thickness according to its location, being thickest upon
the soles of the feet, the palms of the hands, the back, and the outer
portions of the thighs. Its basis is a dense network of elastic fibers,
among which are intricately mingled minute blood-vessels, nerve fibers,
and lymphatic or absorbent vessels. These are most numerous near the
upper surface, and are arranged in loops upon little elevations called
papillæ. In the palms of the hands and upon the soles of the feet these
papillæ are disposed in rows with so much regularity as to give to those
parts a minutely furrowed appearance.
The skin also contains little sacs, or follicles, in which the hairs
originate. In its deeper portions are found two kinds of glands;
sebaceous glands for the secretion of sebaceous or fatty matter to
lubricate the skin, and the perspiratory or sweat glands, the latter of
which will receive a more definite description shortly.
=The Cuticle, or Epidermis.=—At the upper portion of the true skin, new
cells are being constantly formed, which become old in a short time and
are pressed outward by the formation beneath them of other new cells.
The old cells become shriveled and flattened as they grow older, and by
a continuation of the process described, numerous layers of cells are
formed upon the surface of the true skin, the lowest of which is
composed of newly formed cells, while the uppermost one is made up of
dessicated cells having more the appearance of horny scales than of
cells. These several layers constitute the epidermis, or outer skin. It
is totally devoid of sensibility, and has no blood-vessels. It is, in
fact, dead, and is useful only as a protection to living parts beneath.
Scattered among the cells of the epidermis are colored cells, which give
to the skin its proper color. In the Caucasian race, these cells are
few; in the negro, they are abundant; while in the albino, they are
wholly absent.
=The Sweat Glands.=—A close examination of the little ridges found upon
the palms of the hands, by the aid of a small magnifying glass, will
reveal what appear to be fine transverse lines crossing the ridges at
short intervals. A still closer inspection shows that the apparent lines
are really minute openings, guarded by delicate valves. These are the
mouths of the perspiratory ducts, which convey to the surface the
product of the sweat glands. The gland itself is merely a coiled tube,
already described as situated deep down in the true skin, and is
surrounded with a network of blood-vessels. The duct is simply a
continuation of the same tube upward through the cuticle to the surface.
It passes out upon the surface of the skin obliquely, thus leaving a
small portion of the cuticle overlapping its orifice, forming a sort of
valve.
The number of these delicate glands is enormous. It has been carefully
estimated to be about 2,300,000 in a single individual. The length of
each is about one-fifteenth of an inch, making their aggregate length
about two and one-half miles.
=The Mucous Membranes.=—All cavities in the body which communicate with
the surface by openings are lined with a membrane which is called
mucous, from the character of its secretion. The mucous membranes are
continuous with the skin at the natural openings of the body, and very
closely resemble it in structure, being formed of several layers, like
the skin, and having a superficial portion made up of layers formed by
the deeper tissues. Mucous membrane forms the lining of the air-passages
and lungs, of the whole alimentary canal, and of the urinal and genital
organs. Its extent in the lungs alone has been estimated by scientists
at 1400 square feet, or more than seventeen times the whole extent of
the skin.
=Functions of the Skin.=—The skin performs a number of very important
offices for the body. Perhaps the most important is that of excretion.
Each of its millions of sweat glands is actively and constantly engaged
in separating from the blood impurities which would destroy life if
retained. These foul products are poured out through a corresponding
number of minute sewers, and deposited upon the surface of the body to
the amount of several ounces each day, or several pounds, if the whole
perspiration be included in the estimate, as is commonly done.
The skin is also an organ of respiration; it absorbs oxygen, and exhales
carbonic acid gas, with other poisonous gases. The amount of respiratory
labor performed by the skin is about one-sixtieth of that done by the
lungs. In some of the lower animals, the whole work of respiration is
performed by the skin. In the common frog, the respiratory action of the
skin and of the lungs is about equal.
Another important office of the cutaneous tissue is absorption. The
absorption of oxygen has been already referred to; but it absorbs
liquids as well as gases, and to a much greater extent. By immersion in
a warm bath for some time, the weight of the body may be very
considerably increased. Dr. Watson, an English physician of note,
reports the case of a boy whose weight increased nine pounds in
twenty-four hours solely by cutaneous absorption of moisture from the
air. This extraordinary action was occasioned by disease. Seamen, when
deprived of fresh water, quench their thirst by wetting their clothing
with sea-water, the aqueous portion of which is absorbed by the skin.
The lymphatic vessels are believed to be the principal agents in
absorption.
Another remarkable function of the skin is the regulation of
temperature. By its density and non-conducting property it prevents the
escape of necessary heat to a considerable degree. But when the amount
of heat generated in the body becomes excessive, either from abnormal
vital activities, or by exposure to external heat, the skin relieves the
suffering tissues by favoring the escape of heat. This desirable end is
attained through the evaporation of the moisture poured out upon the
surface by the perspiratory glands.
It has been estimated that the evaporation of water from the cutaneous
surface and the mucous membrane of the lungs occasions the loss each
minute of sufficient heat to raise a pint of water 100° F. in
temperature. This is certainly a powerful cooling process.
Lastly, we mention as a further function of the skin, and one which is
not the least in importance, its utility as a sensitive surface. It is a
well-established physiological fact that the mind is only a reflection
of impressions received from without, or at least that its character is
largely determined by the nature of the impressions made upon its organs
of sensibility. The skin is the organ of touch, and the various
modifications of tactile sensibility. It is the most extensive organ of
sensibility in the body, and is very closely connected with all the
great nerve centers, so that it is perhaps the most efficient means
through which to affect the general nervous system. Its intimate
sympathy with internal organs is shown in a great number of diseases in
which this organ evidently suffers on account of disability of some
other one.
=Functions of the Mucous Membranes.=—The functions of the mucous
membranes are strictly analogous to those of the skin. Like the latter
organ, a mucous membrane excretes and absorbs. It eliminates foul
matters, and absorbs useful substances in a fluid state.
The importance of the functions of the skin is shown by the fact that a
person quickly dies when its action is interrupted. A coat of varnish or
caoutchouc, applied over the whole skin, will kill a man almost as
quickly as a fatal dose of strychnia. In experiments upon animals,
horses, dogs, and other animals have been killed by obstructing the
action of the skin by some similar means. A little boy was once killed
by covering him with gold leaf to make him represent an angel at a great
celebration.
The offensive odor of the perspiration, and the characteristic smell of
the sweat-soiled underclothing of a tobacco user, are facts which well
attest the value of the cutaneous functions in removing impurities from
the body.