PHYSIOLOGICAL ANATOMY
THE SKIN.
Through digestion and respiration, the blood is continually supplied
with material for its renewal; and, while the nutritive constituents of
the food are retained to promote the growth of the body, those which are
useless or injurious are in various ways expelled. There are, perhaps,
few parts of the body more actively concerned in this removal than the
skin.
[Illustration: Fig. 46: An ideal
view of the papillae. 1, 1. Cutis vera.
2.2. Papillary layer. 3, 3. Arteries of the papillae.
4, 4. Nerves of the papillae. 5, 5. Veins of the papillae.]
The skin is a membranous envelope covering the entire body. It consists
of two layers, termed the Cutis Vera, or true skin, and the Epidermis,
or cuticle. The _Cutis Vera_ is composed of fibers similar to those of
the cellular tissue. It consists of white and yellow fibers, which are
more densely woven near the surface than deeper in the structure; the
white give strength, the yellow strength and elasticity combined. The
true skin may be divided into two layers, differing in their
characteristics, and termed respectively the superficial or papillary
layer, and the deep or fibrous layer. Upon the external surface, are
little conical prominences, known as _papillae_. The papillae are
irregularly distributed over the body, in some parts being smaller and
more numerous than in others, as on the finger-ends, where their summits
are so intimately connected as to form a tolerably smooth surface. It is
owing to their perfect development, that the finger-tips are adapted to
receive the most delicate impressions of touch. Although every part of
the skin is sensitive, yet the papillae are extremely so, for they are
the principal means through which the impressions of objects are
communicated. Each papilla not only contains a minute vein and artery,
but it also incloses a loop of sensitive nerves. When the body is
exposed to cold, these papillae can be more distinctly seen in the form
of prominences, commonly known as "goose-pimples."
[Illustration: Fig. 47.
A section of the skin, showing its arteries and
veins. A, A. Arterial branches. B, B. Capillaries
in which the branches terminate. C. The venous
trunk into which the blood from the capillaries
flows.]
The internal, or fibrous layer of the skin, contains numerous
depressions, each of which furnishes a receptacle for fat. While the
skin is supplied with a complete net-work of arteries, veins, and
nerves, which make it sensitive to the slightest touch, it also contains
numerous lymphatic vessels, so minute that they are invisible to the
naked eye.
Among the agents adapted for expelling the excretions from the system,
few surpass the _Sudoriferous Glands_. These are minute organs which
wind in and out over the whole extent of the true skin, and secrete the
perspiration. Though much of it passes off as insensible transpiration,
yet it often accumulates in drops of sweat, during long-continued
exercise or exposure to a high temperature. The office of the
perspiration is two-fold. It removes noxious matter from the system, and
diminishes animal heat, and thereby equalizes the temperature of the
body. It also renders the skin soft and pliable, thus better adapting it
to the movements of the muscles. The _Sebaceous Glands_, which are
placed in the true skin, are less abundant where the sudoriferous glands
are most numerous, and _vice versa_. Here, as elsewhere, nature acts
with systematic and intelligent design. The perspiratory glands are
distributed where they are most needed,--in the eyelids, serving as
lubricators; in the ear passages, to produce the _cerumen_, or wax,
which prevents the intrusion of small insects; and in the scalp, to
supply the hair with its natural pomatum.
[Illustration: Fig. 48.
A perspiratory gland, highly
magnified. 1, 1. The gland. 2, 2.
Excretory ducts uniting to form
a tube which tortuously perforates
the cuticle at 3, and opens
obliquely on its surface at 4.]
[Illustration: Fig. 49.
A representation of oil-tubes from the scalp
and nose.]
[Illustration: Fig. 50.
Anatomy of the skin. 5, 5. Cutis vera (true skin).
4, 4. Nervous tissue. 3, 3. Sensitive layer in which are
seen the nerves. 2, 2. The layer containing pigment
cells. 1, 1. Epidermis (cuticle).]
The _Epidermis_, or _Cuticle_, so called because it is _placed upon the
skin,_ is the outer layer of the skin. Since it is entirely destitute of
nerves and blood-vessels, it is not sensitive. Like the cutis vera, it
has two surfaces composed of layers. The internal, or _Rete Mucosum,_
which is made up chiefly of pigment cells, is adapted to the
irregularities of the cutis vera, and sends prolongations into all its
glandular follicles. The external surface, or epidermis proper, is
elastic, destitute of coloring matter, and consists of mere horny
scales. As soon as dry, they are removed in the form of scurf, and
replaced by new ones from the cutis vera. These scales may be removed by
a wet-sheet pack, or by friction. The cuticle is constantly undergoing
renewal. This layer serves to cover and protect the nervous tissue of
the true skin beneath. We may here observe that the cuticle contains the
pigment for coloring the skin. In dark races, as the negro, the cuticle
is very thick and filled with black pigment. The radiation of animal
heat is dependent upon the thickness and color of this cuticle. Thus, in
the dark races, the pigment cells are most numerous, and in proportion
as the skin is dark or fair do we find these cells in greater or lesser
abundance. The skin of the Albino is of pearly whiteness, devoid even of
the pink or brown tint which that of the European always possesses. This
peculiarity must be attributed to the absence of pigment cells which,
when present, always present a more or less dark color. The theory that
_climate_ alone is capable of producing all these diversities is simply
absurd. The Esquimaux, who live in Greenland and the arctic regions of
America, are remarkable for the darkness of their complexion. Humboldt
remarks that the American tribes of the tropical regions have no darker
skin than the mountaineers of the temperate zone. Climate may _modify_
the complexion, but it cannot _make_ it.
[Illustration: Fig. 51.
Structure of the human hair. _A_. External surface of the shaft, showing the
transverse striae and jagged boundary, caused by the imbrications of the scaly
cortex. _B_. Longitudinal section of the shaft, showing the fibrous character of
the medullary substance, and the arrangement of the pigmentary matter. _C_.
Transverse sections, showing the distinction between the cortical and medullary
substances, and the central collection of pigmentary matter, sometimes found in
the latter. Magnified 310 diameters.]
_Hairs_ are horny appendages of the skin, and, with the exception of the
hands, the soles of the feet, the backs of the fingers and toes, between
the last joint and the nail, and the upper eyelids, are distributed more
or less abundantly over every part of the surface of the body. Over the
greater part of the surface the hairs are very minute, and in some
places are not actually apparent above the level of the skin; but the
hair of the head, when permitted to reach its full growth, attains a
length of from twenty inches to a yard, and, in rare instances, even six
feet. A hair may be divided into a middle portion, or _shaft_, and two
extremities; a peripheral extremity, called the _point;_ and a central
extremity, inclosed within the hair sac, or follicle, termed the _root_.
The root is somewhat greater in diameter than the shaft, and cylindrical
in form, while its lower part expands into an oval mass, called the
_bulb_. The shaft of the hair is not often perfectly cylindrical, but is
more or less flattened, which circumstance gives rise to waving and
curling hair; and, when the flattening is spiral in direction, the
curling will be very great. A hair is composed of three different layers
of cell-tissues: a loose, cellulated substance, which occupies its
center, and constitutes the _medulla_, or pith; the fibrous tissue,
which incloses the medulla, and forms the chief bulk of the hair; and a
thin layer, which envelops this fibrous structure, and forms the smooth
surface of the hair. The medulla is absent in the downy hairs, but in
the coarser class it is always present, especially in white hair. The
color of hair is due partly to the granules and partly to an
inter-granular substance, which occupies the interstices of the granules
and the fibers. The quantity of hair varies according to the proximity
and condition of the follicles. The average number of hairs of the head
may be stated at 1,000 in a superficial square inch; and, as the surface
of the scalp has an area of about one hundred and twenty superficial
square inches, the average number of hairs on the entire head is
120,000. The hair possesses great durability, as is evinced by its
endurance of chemical processes, and by its discovery, in the tombs of
mummies more than two thousand years old. The hair is remarkable for its
elasticity and strength. Hair is found to differ materially from horn in
its chemical composition. According to Vauquelin, its constituents are
animal matter, a greenish-black oil, a white, concrete oil, phosphate of
lime, a trace of carbonate of lime, oxide of manganese, iron, sulphur,
and silex. Red hair contains a reddish oil, a large proportion of
sulphur, and a small quantity of iron. White hair contains a white oil,
and phosphate of magnesia. It has been supposed that hair grows after
death, but this theory was probably due to the lengthening of the hair
by the absorption of moisture from the body or atmosphere.
The _nails_ constitute another class of appendages of the skin. They
consist of thin plates of horny tissue, having a root, a body, and a
free extremity. The root, as well as the lateral portion, is implanted
in the skin, and has a thin margin which is received into a groove of
the true skin. The under surface is furrowed, while the upper is
comparatively smooth. The nails grow in the same manner as the cuticle.
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