THE SKIN, ITS APPENDAGES AND ITS FUNCTION.
The whole exterior surface of the body is covered by the skin, an
excreting and absorbing organ, which serves as a protection to the
parts beneath and is also the organ of touch. It has two layers,
a superficial and a deep. The superficial layer, the epidermis or
cuticle, is composed wholly of epithelial cells, of which the deepest
layer is columnar and moulded upon the papillary layer of the derma,
while the intermediate layers are more rounded and the surface ones
flat. The deepest layer also contains the skin pigment, which causes
the variation in shade between the Indian, the negro, and the white
man. Below the epidermis, which is chiefly protective, is the tough,
elastic, and flexible tissue of the derma or true skin, in which are
vested most of the activities of the skin. Its surface is covered
with papillæ, which are more numerous in the more sensitive parts.
Each papilla contains one or more capillary loops and one or more
nerve fibers, while some terminate in an oval body known as a tactile
corpuscle. Beneath the papillæ is the reticular layer, composed of
interlacing bands of fibrous tissue and containing blood-vessels,
lymphatics, and nerves, as well as unstriped muscle fibers where hair
is present.
[Illustration: FIG. 11.--Vertical section of skin.]
At the apertures of the body the skin stops and is replaced by _mucous
membrane_, an integument of greater delicacy but which consists
fundamentally of the same two layers, a superficial, bloodless
epithelium and a deep fibrous derma. It is continuous with the skin,
but is much redder and more sensitive and bleeds more easily. The
passages and cavities that it lines, unlike those lined by serous
membranes, communicate with the exterior of the body and are for that
reason protected against contact with foreign substances by mucus,
which is thicker and more sticky than the lymph that moistens the
endothelium found on serous surfaces. Mucous membrane is found in the
alimentary canal, the respiratory tract, and the genito-urinary tract.
In cavities, like the stomach and intestines, which are subject to
variations in capacity, it is thrown into folds or rugæ. The mucus is
secreted by small glands in the membrane.
=Appendages of the Skin.=--The skin has various appendages. On the
dorsal surface of the last phalanges of the fingers and toes are
flattened and horny modifications of epithelium, the _nails_. They have
a root embedded in a groove of skin by which they grow in length and a
vascular matrix of derma beneath them which gives growth in thickness.
To their growth in length there seems to be no limit.
The _hairs_ also, which occur all over the body, except on the palms
of the hands and the soles of the feet, are a modification of the
epithelium. Each hair has a bulbous root springing from an involution
in the epidermis and derma called the _hair follicle_, into which one
or two sebaceous glands empty. It is raised by involuntary muscle
fibers and grows by constant additions to the surface by which it is
attached. This growth seems, however, to be limited, and when its term
is reached the hair falls out and is replaced by another. The horny
epithelial cells that go to form the hair contain the pigment that
gives it its color.
[Illustration: FIG. 12.--Skin and longitudinal section of hair: _a_,
Epidermis; _b_, corium; _c_, sebaceous gland; _d_, fibrous root-sheath;
_e_, glassy membrane; _f_, outer root-sheath; _g_, inner root-sheath;
_h_, expanded bulbous end of hair; _i_, papilla of hair; _j_, arrector
pili; _k_, adipose tissue. (Leroy)]
Like the hairs, the _sebaceous glands_ are situated in all parts of
the body except the palms of the hands and the soles of the feet. They
lie in the papillary layer and empty into the hair follicles, except
occasionally, when they empty directly upon the surface of the skin.
They secrete an oily substance, _sebum_, the débris resulting from the
degeneration of the epithelial cells of the gland itself, which serves
to keep the hair glossy and the skin soft and flexible.
The _sweat glands_, on the other hand, are more frequent on the palms
and soles and though sometimes found in the derma are usually situated
lower down in the subcutaneous cellular tissue. They are least numerous
on the back and neck. Coiled up in the lower layers of the skin, they
discharge the sweat through a spiral excretory duct upon its free
surface.
The _sweat_ is a clear, colorless, watery fluid with a salty taste,
an alkaline reaction, and a characteristic odor that varies with
the individual. If very scanty, it may be acid in reaction. Besides
water it contains a small percentage of solids, as inorganic salts,
especially sodium chloride, fatty acids, neutral fats, and at times,
especially in some diseases of the kidneys, urea, that is, the
end-products of the metabolism of starches and fats chiefly. There
is usually also some carbon dioxide, whence the expression cutaneous
respiration.
The sweat serves to keep the skin moist and in good condition, to
remove outworn and poisonous or irritating matters, and to regulate
the temperature. As a rule it evaporates upon reaching the surface, in
which case it is known as invisible or insensible perspiration, but if
conditions of the atmosphere are not favorable to prompt evaporation,
as when the air is damp, the skin becomes damp and there is visible
perspiration.
Though an abundant supply of blood increases the action of the sweat
glands, they are regulated by definite secretory nerves rather than
by the vasomotor nerves. In a cold sweat the action is probably due
to some disturbance of the nerve supply without increase of the blood
supply. Ordinarily perspiring is a reflex act due to the stimulation
of the afferent cutaneous nerves, as by the application of heat, but
sometimes, as in cases of strong emotions, involuntary impulses are
sent from the brain to the spinal centers and so arouse the action
of the glands. Atropin has the power of preventing the secretion of
sweat by paralyzing the terminations of the secretory nerves, while
pilocarpin produces an opposite effect in a similar way.
On account of these sweat glands the skin becomes next in importance
after the kidneys in the excretion of waste products. The quantity of
sweat excreted varies greatly and is hard to measure. It is influenced
by the temperature and humidity of the surrounding air, by the nature
and quantity of food and drink consumed, by the amount of exercise,
the relative activity of other organs, especially the kidneys, and by
certain mental conditions. The hotter it is, the greater the amount of
perspiration. In damp weather there may be less perspiration, but it
does not evaporate and is therefore more in evidence.
Ordinarily man has a temperature of 98.6°. The source of this body heat
or temperature is the general body metabolism, muscular activity, and
activity of the glands, especially of the liver, which is constantly
active, the blood in the hepatic vein being warmer than that in any
other part of the body. The tissue of the brain also is said to be
warmer than the surrounding blood, and the heart and respiratory
muscles, which are in constant activity, are responsible for much of
the body heat. The amount of heat generated in the body, therefore,
varies at different times, according as a person is awake or asleep,
quiet or active.
=Temperature Regulation.=--The temperature is regulated by variations
in the production and loss of heat, less being known of its production
than of its loss. It has been calculated that four-fifths of the energy
of the body is converted into heat, one-fifth into work. As the minimum
amount of heat produced in twenty-four hours is sufficient to raise 10
gallons of water from 0° to boiling-point, it is evident that if there
were not some way for the escape of much of this heat the body would
become hotter and hotter and finally destroy itself. The temperature,
however, except on the surface, is uniform, heat being lost as fast
as it is produced. For, although oxidation at any point raises the
heat of the blood at the point, this heat is carried by the blood to
other parts, to which the surplus is given up, while blood cooled in
the skin goes to the hotter inward parts to cool them and be warmed
itself. In fact, heat is expended by conduction and radiation, through
respiration, perspiration, and heat given to the urine and fæces. It
is, therefore, largely, 75 to 80 per cent., carried off through the
skin and the lungs; 60 to 70 per cent. is lost by radiation to the
air and other bodies with which the body comes in contact; 20 to 30
per cent. is lost by the evaporation of sweat, 4 to 8 per cent. by
the warming of expired air, urine and feces, and 1 to 2 per cent.
by cold food that is taken in. Radiation acts more favorably where
the surroundings are cool and the air in motion, as on a breezy day.
Conduction is carried on best where the surrounding air is cool,
especially if it is moist, for moist air is a better conductor of heat
than dry air. Evaporation is very important in hot weather or where men
work in hot air.
Even in health the temperature may range from 98.6° to 99.5°, and a
degree or two below or above is not dangerous. When a person first
gets up in the morning his temperature is apt to be subnormal, but
after food and exercise have been taken it becomes normal and stays
so till the end of the day, when, if the person is tired, it may go
up a little. If a person is tired out, the temperature is apt to be
subnormal. There is also in the body what is called the vital tide,
which is highest afternoon and evening and lowest in the morning.
The rate of production of heat varies greatly in different people. One
person uses a certain amount of tissue more quickly than another, that
is, he lives faster. Moreover, size makes a difference in that a small
body has more surface to its weight than a large one and so has to
produce the same amount of heat at a faster rate in order to maintain
the right temperature. Taking food increases heat, probably because
of the muscular effort needed to eat it. Muscular work is another
factor. And finally the whole matter of heat production seems to be
under the control of the nervous system. Not much is known on this
point except that there is a heat center in the medulla which plays an
important part in heat production and whose influence is seen where
the temperature shoots way up in disease just before death. It is now
thought that fever is due to a disturbance of this nervous mechanism,
though just what the disturbance is is not known.
_Fever_ is a condition of increased bodily temperature, due to
increased production or to decreased loss of heat. As a rule, in all
fevers the metabolic changes in the body are increased. Hence the
patient becomes emaciated in a long fever. The frequent increase in the
amount of urea during fever shows an increase in protein metabolism.
The temperature in fevers rises as high as 106° and in sunstroke
sometimes to 110°. Except in sunstroke a higher temperature than 106°
generally means death. Subnormal temperature is due to a decrease in
the bodily metabolism and so to lessened heat production. As a rule,
if the functions are all active, especially that of the sweat glands,
a person can be exposed to severe heat without the temperature being
affected, though sometimes on a hot summer day it may be up half to one
degree. The cause of heat-stroke with its high fever is unknown, but
probably it is due to some effect on the heat center in the brain. Heat
prostration is also due to prolonged exposure to heat, but is generally
accompanied by a subnormal temperature. The effect of cold, as in
freezing, is to diminish all the metabolic activities of the body. The
temperature can be artificially regulated more or less by variations of
food, varying amounts of exercise, by drugs, etc.
=Sense of Touch.=--Before passing on to a discussion of the individual
parts, a few words might well be said of the sense of touch, since that
is general and resides largely in the skin, whose other functions have
just been described. It may be regarded as the form from which all
the other special senses have developed, certain portions of the body
having become more sensitive than others to certain vibrations, as the
eye to those of light. The internal organs probably have little sense
of touch.
[Illustration]
[Illustration: FIGS. 13, 14.--Meissner’s corpuscle from man; ×750.
(Böhm, Davidoff, and Huber.)]
Touch is useful only within arm’s reach but there gives one a sense of
space that sight does not give. It is practically determined by the
_touch corpuscles_, which are found in the skin over almost the entire
body, though they are more numerous in some places than in others,
the distribution of the corpuscles determining the sensitiveness of
the skin. These touch corpuscles are protoplasmic bodies containing
nuclei, about which are entwined filaments from the cutaneous nerves.
Where the corpuscles are absent the filaments of the cutaneous nerves
themselves play an important part. The finger tips have a very delicate
sense of touch and the tip of the tongue is the most sensitive part of
the body. Hence spaces in the mouth seem larger than elsewhere. By
the transmission of sensations of touch to the brain the sensation is
localized and the tactile sensation becomes a tactile perception.
There are three main divisions of the sense of touch: 1. sensations of
touch proper or tactile sensation; 2. sensations of temperature, and
3. sensations of pain. The temperature sense is the transmission by
the skin of sensations not so much of a certain degree of heat or cold
as of the difference between the temperature of an object and that of
the skin. The longer an object is in contact with the skin, the less
conscious the person is of it, not only because it becomes of the
same temperature, but also because he becomes accustomed to it. There
also seem to be in the skin, besides the touch corpuscles, two other
terminal organs with separate nerve fibers, the one for detecting heat,
the other cold; for there are places on the body where heat can be
detected and cold cannot, and _vice versa_.
Sensations of pain may be merely an exaggeration of tactile sensation,
as in too hard pressure or too great heat, but there seems to be also
a sensation of pain in the skin. All organs are said to have common
sensibility to pain and any exaggeration of this sensibility causes a
sensation of pain. All the special senses require a certain amount of
judgment in the interpretation of the sensations they convey.