PHYSIOLOGY OF THE SKIN.
637. The skin invests the whole of the external surface of the body,
following all its prominences and curves, and gives protection to all
the organs it encloses, while each of its several parts has a distinct
use.
638. The cuticle is insensible, and serves as a sheath of protection
to the highly sensitive skin (_cutis vera_) situated beneath it. The
latter feels; but the former blunts the impression which occasions
feeling. In some situations, the cuticle is so dense and thick, as
wholly to exclude ordinary impressions. Of this we see an example in
the ends of the fingers, where the hard and dense nail is the cuticle
modified for the purpose referred to. Were the nervous tissue of the
true skin not thus protected, every sensation would be so acute as to
be unpleasant, and contact with external bodies would cause pain.
639. The cuticle, also, prevents disease, by impeding the evaporation
of the fluids of the true skin, and the absorption of the poisonous
vapors, which necessarily attend various employments. It, however,
affords protection to the system only when unbroken, and then, to the
greatest degree, when covered with a proper amount of oily secretion
from the oil-glands.
640. The cuticle is, originally, a transparent fluid, exuded by the
blood-vessels, and distributed as a thin layer on the surface of the
true skin. While successive layers are formed on the exterior of the
true skin, the external cuticular layers are converted into dry,
flattened scales, by the evaporation of their fluid contents. The
thickness of the cuticle is formed mainly from these scales.
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637-656. _Give the physiology of the skin._ 637. What is said of the
skin? 638. Give a function of the cuticle. Does it vary in thickness
on different parts of the body? Give examples. 630. Mention another
use of the cuticle. 640. What is the cuticle originally?
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641. The cuticle is, therefore, undergoing a constant process of
formation and growth at its under part, to compensate for the wear
that is taking place continually on its surface. A proper thickness of
the cuticle is in this manner preserved; the faculty of sensation and
that of touch are properly regulated; the places of the little scales,
which are continually falling off under the united influence of
friction and ablution, are supplied; and an action necessary, not
merely to the health of the skin, but to that of the entire body, is
established.
642. Whenever the cuticle is exposed to moderate and repeated
friction, it becomes thicker and tougher, as may be seen in the
cuticle of the lady's finger that plies the needle and in the hard or
callous appearance of the hands of farmers masons, and other
mechanics. This enables them to handle the utensils and materials used
in their vocations without pain or inconvenience.
_Observations._ 1st. When the joints of the feet are subjected to
moderate and continued pressure or friction, frequently one or more of
the papillae enlarge. This is accompanied with a thickening of the
layers of the cuticle, which is termed a "callosity," or "corn." These
thickened layers of the cuticle are broad at the top and narrow at the
bottom, and the enlarged mass is conical, with the point innermost.
When pressed upon by a tight shoe, these sensitive papillae cause
pain.
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How is the thickness of the cuticle mainly formed? 641. Describe the
changes of this membrane. Show the necessity of this constant growth.
642. How does moderate and repeated friction affect the cuticle? Give
examples. What is the benefit derived from having the cuticle thus
changed? What is the result if the joints of the feet are subjected to
moderate and continued pressure? What is the form of a "corn"?
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2d. To remove these painful excrescences, take a thick piece of soft
leather, somewhat larger than the corn; in the centre punch a hole of
the size of the summit of the corn, spread the leather with adhesive
plaster, and apply it around the corn. The hole in the leather may be
filled with a paste made of soda and soap, on going to bed. In the
morning, remove it, and wash with warm water. Repeat this for several
successive nights, and the corn will be removed. The only precaution
is, not to repeat the application so as to cause pain.
643. Let a person unaccustomed to manual labor, trundle the hand-cart,
or row a boat, for several successive hours, and the cuticle upon the
palms of the hands, instead of becoming thicker by use, is frequently
separated from the subjacent tissues, by an effusion of serum,
(water,) thrown out by the vessels of the true skin. Had the friction
been moderate, and applied at regular intervals, instead of blisters
being formed upon the inside of the hands, material would have been
thrown out to form new layers upon the lower surface of the cuticle.
644. The cuticle is interesting to us in another point of view,
as being the seat of the color of the skin. The difference of
color between the blonde and the brunette, the European and the
African, lies in the cuticle;--in the deeper, and softer, and
newly-formed layers of that structure. In the whitest skin, the
cells of the cuticle always contain more or less of a peculiar
pigment, incorporated with the elementary granules which enter
into their composition. In the white races, the pigmentary tint
is extremely slight, and less in winter than in the summer season.
In the darker races, on the contrary, it is deep and strongly
marked.
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How can they be removed? What precaution is given? 643. Explain why
those persons unaccustomed to labor, blister their hands in rowing a
boat or performing ordinary manual employment for several successive
hours. 644. In what other point of view is the cuticle interesting? In
what part of it do we find the coloring matter?
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645. The various tints of color exhibited by mankind, are, therefore,
referable to the amount of coloring principle contained within the
elementary granules of the cuticle, and their consequent depth of hue.
In the negro, the granules are more or less black; in the European of
the south, they are amber-colored; and in the inhabitants of the
north, they are pale and almost colorless.
646. Color of the skin has relation to energy in its action; thus, in
the equatorial region, where light and heat are most powerful, the
skin is stimulated by these agents to vigorous action, and color is
very deep; while in the temperate regions, where light and heat are
not so intense, the lungs, liver, and kidneys relieve the skin of part
of its duties. The colored layer of the cuticle has been called the
_re'te mu-co'sum_, (mucous coat of the skin,) and described as a
distinct layer by many physiologists.
_Observation._ "The various coloring of the inner layer of the cuticle
gives to some animals their varied hues; the serpent, the frog, the
lizard, and some fishes have a splendor of hue almost equal to
polished metal. The gold-fish and the dolphin owe their difference of
color and the brilliancy of their hues to the color of this layer of
the skin."
647. The nerves of the skin are the organs of the sense of touch and
feeling. Through them we receive many impressions that enhance our
pleasures, as the grateful sensations imparted by the cooling breeze
in a warm day. In consequence of their sensitiveness, we are
individually protected, by being admonished of the proximity of
destructive agents.
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In what season of the year is the coloring matter less in the white
race? 645. To what is the color of the skin referable? 646. Why have
the races of the torrid zone darker complexions than those of the
temperate or frigid zones? What is this colored layer called by many
physiologists? To what is the different hues in animals owing? 647. Of
what use are the nerves of the skin?
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_Illustration._ A man who had been afflicted some years with a severe
disease of a portion of the brain and spinal cord, was deprived of
feeling in the lower extremities. He was directed by his attending
physician to use a warm footbath. Intending to follow the directions
given him, he immersed his feet in boiling water, which he supposed of
a proper temperature. While his feet were immersed in the water, he
experienced no sensation of an unpleasant nature. On withdrawing them,
he was astonished to find the cuticle separated from the other
tissues, by the effusion of serum, and thus producing a blister over
the whole surface.
648. Portions of the skin would suffer every day, were it not for the
sentinel-like care exercised by the nerves, by which all impressions
are transmitted to the brain. As the skin is continually exposed to
the influence of destructive agents, it is important that the nerves,
provided for its protection, should be kept in a healthy state.
649. A large proportion of the waste of the body passes through the
outlets of the skin; some portions in the form of oil, others in the
form of water and carbonic acid.
650. The oil-glands secrete an oil, partly free and diffused, and
partly mixed with albumen. When the cells are fully formed, that is,
fully distended, they yield their contents, and the fluid matter they
contain is set free, and passes along the tubes to the surface; this
fluid matter constitutes the oily element of the economy of the skin.
651. The uses of the unctuous product of the oil-glands are twofold:
1st. The protection; 2d. The removal of waste matter from the system.
In the exercise of these offices the oily substance is diffused over
those parts of the skin which are naturally exposed to vicissitudes of
temperature and moisture,--as the nose, face, and head;--to the
injurious attrition of contiguous surfaces,--as the flexures of
joints;--or the contact of acrid fluids,--as in the excoriations to
which infants are liable.
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Give the illustration. 648. Why is it necessary that the cutaneous
nerves be kept in a healthy state? 649. Through what membrane does a
large proportion of the waste material of the system pass? 650. What
is the function of the oil-glands? 651. What are the uses of the oily
product of these glands?
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652. The oil of the unctuous substance is the principal agent in
effecting these purposes: 1st. It prevents the evaporation or
congelation of the water of the cuticle, which would cause it to
become parched and peel off, thus leaving the sensitive skin exposed.
2d. It affords a soft medium to the contact of moving substances. 3d.
It repels moisture and fluids. 4th. The action of these glands removes
the waste atoms and purifies the blood.
653. In considering the purpose of the oily matter of the skin, there
are two situations in which it deserves especial remark. 1st. Along
the edges of the eyelids, where it is poured out in considerable
quantity. Here, it is the means of confining the tears and moisture of
the eyes within the lids, defending the skin from the irritation of
that fluid, and preventing the adhesion of the lids, which is liable
to occur upon slight inflammation. 2d. In the ears, where the unctuous
wax not only preserves the membrane of the drum and the passage of the
ear moist, but also, by its bitterness, prevents the intrusion of
small insects.
654. The use of the perspiratory glands is to separate from the blood
that portion of the waste matter which is carried off through the skin
in the form of vapor. Sanctorius, a celebrated medical writer, daily,
for thirty years, weighed himself, his food, and excretions. He
estimated that _five_ of every _eight_ pounds of food and drink passed
from the system through the many outlets upon the skin. Many place the
estimate much lower. All physiologists agree that from twenty to forty
ounces of matter pass off from the skin of an adult every twenty-four
hours.
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652. What prevents the evaporation of the water of the cuticle? Give
its 2d use. Its 3d. Its 4th. 653. What is said in reference to the
distribution of the oily matter along the edges of the eyelids? In the
ears? 654. Of what use are the perspiratory glands? How long did
Sanctorius daily weigh his food, to ascertain the amount of secretion
that passed through the skin?
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655. The average amount of perspiration is about thirty ounces; and it
passes off in such minute portions, and mixes so rapidly with the
surrounding air, that it is not perceived. For this reason, it is
called _insensible_ perspiration. When this excretion is increased, it
forms into drops, and is called _sensible_ perspiration. The following
experiments prove the existence of this excretion from the skin.
_Experiments._ 1st. Take a cold bell-glass, or any glass vessel large
enough to admit the hand, and introduce it perfectly dry; at the same
time close the mouth by winding a napkin about the wrist; in a short
time, the insensible perspiration from the hand, will be seen
deposited on the inside of the glass. At first, the deposit is in the
form of mist; but, if the experiment be continued a sufficient time,
it will collect in drops.
2d. Hold the apparently dry hand near a looking-glass, and the
invisible vapor will soon be condensed, and cover the glass with a
slight dew.
656. It is important that this excretion be maintained with steadiness
and regularity. When the action of the perspiratory glands is
suppressed, all the vessels of the different organs will suffer
materially, and become diseased, by the redundant waste matter that
should be carried from the system. If a person is vigorous, the action
of the organs, whose functions are similar to those of the skin, as
channels for the exit of waste matter, will be increased, and thus
relieve the diseased state of the body. But the over-taxing of these
organs, to relieve the system, often produces a diseased action in
themselves.
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What were his conclusions? 655. What is the average amount of
perspiration every twenty-four hours? What is insensible perspiration?
What is sensible perspiration? How can the existence of the excretion
of the skin be shown? Give the 2d experiment. 656. Why is it important
that these excretions be maintained regularly?
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[Illustration: Fig. 117. 1, 1, The lines, or ridges of the cuticle, cut
perpendicularly. 2, 2, 2, 2, 2, The furrows, or wrinkles of the same. 3,
The cuticle. 4, 4, 4, The colored layer of the cuticle. 5, 5, The cutis
vera. 6, 6, 6, 6, 6, The papillae. 7, 7, Small furrows between the
papillae. 8, 8, 8, 8, The deeper furrows between each couple of the
papillae. 9, 9, Cells filled with fat. 10, 10, 10, The adipose layer, with
numerous fat vesicles. 11, 11, 11, Cellular fibres of the adipose tissue.
12, Two hairs. 13, A perspiratory gland, with its spiral duct. 14,
Another perspiratory gland, with a duct less spiral. 15, 15, Oil-glands
with ducts opening into the sheath of the hair, (12.)]
_Note._--Let the pupil review the anatomy and physiology of the skin
from Fig. 117 or from anatomical outline plate No. 9.