CHAPTER II
THE SKIN
NOTE TO TEACHER.--The experiments should be assigned in turn to the pupils as each chapter is reached: _e.g._ this set of 13 will leave 3 pupils in a class of 39 to stand responsible for each experiment. Each pupil should do the work separately and credit may be given for the best results. Encourage (or require) each pupil to try every experiment and record them in a note book.
_Experiment 1._ (At home or in class.) =Albinism.=--Study a white rabbit as an example of albinism. Does albinism affect only the skin? What evidence that its blood is of normal color?
_Experiment 2._ =Use of Hairs on the Skin.=--Let one pupil rest his hand upon the desk behind him while another touches a hair on his hand with a pencil. He should speak at the moment, if it is felt. Do the hairs increase the sensitiveness of the skin? What was their use with primitive man? Are the hands of all your acquaintances equally hairy? Are the hairs to be classed as rudimentary? Will they disappear? Will the race become baldheaded?
_Experiment 3._ (Home or school.) =Invisible Perspiration.=--Hold a piece of cold glass near the hand or place the cheek near a cold window pane and notice for evidence of moisture. Its source?
_Experiment 4._--=Effect of Evaporation on Temperature.=--Read a thermometer and cover its bulb with a moist cloth. Read again after twenty minutes. Repeat experiment in breeze.
_Experiment 5._ Moisten one hand and allow it to dry. Touch the other hand with it. Explain result.
_Experiment 6._ =Absorbing Power of Fabrics.=--Wet the hands and dry them upon a piece of cotton cloth. Repeat with woolen, linen, and silk. Arrange in list according to readiness in absorbing water.
_Experiment 7._ =Rates of Drying.=--Immerse the cloths in water and hang them up to dry. Test their rates of drying with dry powder or by touch.
_Experiment 8._ =Test Looseness of Weave= of above cloths by measuring the distance pieces of equal length will stretch.
[Illustration: COLORED FIGURE 1.--SECTION OF SKIN (diagram, enlarged 25 times). On the left the connective tissue fibers of the true skin are shown.
In cutis (_c_), or dermis, find capillaries, nerve fibers, fat cells, _two_ sweat glands and ducts, _four_ oil glands (two in section), _two_ hairs, _three_ nerve papillæ, _five_ papillae containing capillaries, _two_ muscles for erecting hairs. In epidermis find flat cells, round cells, and pigment cells.
FIG. 2.--WHERE THE FOOD IS ABSORBED (villus of intestine).
FIG. 3.--WHERE THE FOOD IS USED (cells with lymph spaces).
FIG. 4.--IDEAL SECTION OF MAMMAL.
Compare with organs of man (colored Fig. 6).
_j_, _j_, jaws; _ol_, nerve of smell; _op_, nerve of sight; _b_, brain; _t_, tongue; _ep_, epiglottis; _oe_, gullet; _th_, thymus gland; _lg_, lung; _h_, heart; _l_, liver; _g_, stomach; _s_, spleen; _p_, pancreas; _k_, kidney; _d_, diaphragm; _m_, muscle; _u_, bladder; _ch_, spinal cord; _v_, vertebræ.]
_Experiment 9._ =Does Cotton or Wool protect better from Radiant Heat?=--Lay a thermometer in the sun for ten minutes, first covering it with a woolen cloth. Note change in reading. After it regains first reading, repeat, covering it with a cotton cloth of same weight and texture. Conclusion? Expose wrists or arms to sun for five minutes, one protected by the cotton, the other by the wool. Result? Conclusion?
_Experiment 10._ =Rates of Heat Absorption and Radiation by Different Colors.=--Expose thermometer to sunlight, covered successively by pieces of cloth of same thickness, material, and texture. Use black, blue, red, yellow, and white cloth. Note rise of temperature for equal times in each case; also the fall of temperature for equal times after removal to shade.
_Experiment 11._ =Effects of Dry Powders.=--Prepare two squares from the same piece of leather (_e.g._ an old shoe). Moisten them both, and apply face powder to one. Which dries more quickly? Repeat after oiling them. Powder a portion of the face or arm daily for a week and compare with the clean portion.
_Experiment 12._ =Dissect the kidney= of an ox or sheep, making out the parts mentioned in the text, p. 26.
_Experiment 13._ (In class.) =Emergency Drill.=--Have one pupil wet an imaginary burn on the arm of another, treat it with flour or soda, and bandage. (See text.)
=The Skin has Two Layers.=--The outer layer is called the _epidermis_; it is _thinner, more transparent, and less elastic_ than the inner layer, or _dermis_. The epidermis is composed of epithelial cells packed close together (see colored Fig. 1).
=The dermis=, or inner layer, is a closely woven sheet of _connective tissue_ (colored Fig. 1) containing a great number of _sweat_ and _oil glands_, _roots of hairs_, _blood vessels_, _absorbent vessels_ (lymphatics), and _nerves_ (colored Fig. 1). The dermis is sometimes called the true skin because it is of greater importance than the epidermis. It is united loosely to the underlying organs by a layer of connective tissue. It is in this layer that fat is stored. The upper surface of the dermis rises into a multitude of projections (see colored Fig. 1) called _papil′læ_ (singular, papilla). The epidermis fits closely over them and completely levels up the spaces between them except on the palms and the soles. Here the papillæ are in rows, and there is a fine ridge in the skin above each row of papillæ (Fig. 24). In the papillæ are small loops of blood vessels and sometimes a nerve fiber (colored Fig. 1).
[Illustration: FIG. 20.--EPIDERMIS OF ETHIOPIAN.]
[Illustration: FIG. 21.--EPIDERMIS OF CAUCASIAN.]
=The epidermis= _is composed of a mass of cells_ held together by a cement resembling the white of an egg. The cells near the surface are hard and flattened; those deeper down near the dermis are round and soft (see Fig. 21). These cells are living cells. They are kept alive by the nourishment in the watery portion of the blood which soaks through from the blood tubes in the neighboring papillæ. Hence these cells are growing cells; they subdivide when they reach a certain size, and replace those wearing away at the surface, thus constantly repairing the epidermis. The dry outer cells wear away rapidly. They have no nuclei and are dead cells. The new cells forming beneath push them so far away from the dermis that nourishment no longer reaches them, and they die.
=Pigment.=--The cells in the lower layers of the epidermis contain grains of coloring matter, or pigment. All other cells of the epidermis are transparent; the pigment has the function of absorbing and arresting light. Albinos or animals entirely without pigment have pallid skins and pink eyes (Exp. 1).
=Immigrants from a Cloudy to a Sunny Climate. Adaptation.=--The cells of the deeper tissues can readily be exhausted by the stimulation of too much light. The sunnier the climate, the greater the need of pigment; hence the dark skin of the negro and the blonde skin and hair of the Norwegian. European immigrants to sunny America will grow darker. The Indian’s skin is better suited to our climate than is a fair skin. Brunettes have a better chance for adaptation than blondes. The American type when developed will doubtless be brunette.
=The hair= grows from a pit or follicle (Fig. 22). _Blood vessels and a nerve fiber go to the root_ or bulb from which a hair grows. The hair will grow until this papilla, or bulb, is destroyed (Exp. 2).
[Illustration: FIG. 22.--DEVELOPMENT OF A HAIR AND TWO OIL GLANDS.]
=Adaptation of the scalp to a tight warm covering= is accomplished through the shedding of the hair rendered useless by the covering. It is impossible to stop the growth of superfluous hair unless the hair papillæ are destroyed with an electric needle, such is the vitality of hair; yet many men, by overheating the head and cutting off the circulation with tight hats, destroy much of the hair before reaching middle age. The health of the hair can be restored and its loss be stopped by going bareheaded except in the hot sun or in extremely cold weather. This frees the circulation; cold air and light stimulate the cells of the scalp. Some men wear hats, even at night in summer. The brain needs the protection of the hair. Beard protects the larynx or voice box, which is large and exposed in man. It was also a protection in hunting wild beasts and in war. Compare mane of lion, not possessed by lioness. “Goose-flesh” after a cold bath is caused by the contraction of small muscles (colored Fig. 1), raising the now tiny hairs in an absurdly useless effort to keep the body warm.
=The nails= are dense, thick plates of epidermis growing from a number of papillæ situated in a groove, or fold, of the skin; there are many fine papillæ along the bed from which the nail grows. Since it grows from its under side as well as from the little fold of skin at its root, the nail is thicker at the end than near the root.
[Illustration: FIG. 23.--_A_, DEVELOPMENT OF SWEAT GLAND; _B_, SWEAT TUBE DEVELOPED.]
The =oil glands= empty into the hair follicles (colored Fig. 1). They form an oil from the blood that _keeps the hair glossy and the surface of the skin soft and flexible_ by preventing excessive drying. Hair oil should never be used upon the hair, as the oil soon becomes rancid, and besides causes dust and dirt to stick to the hair.
=The sweat glands= (Fig. 23), like the hair bulbs, are deep in the lowest part of the dermis. _A sweat gland has the form, of a tube coiled into a ball_ (colored Fig. 1). This tube continues as a duct through the two layers of skin, and its opening at the surface is called a _pore_ (Fig. 24). The perspiration evaporates as fast as it flows out through the pores, if the secretion is slow; but if poured out rapidly, it gathers into drops (Exp. 3). The perspiration is chiefly water, containing in solution several salts, including common salt and a trace of a white, crystalline substance called _urea_. The material for the perspiration is furnished by the blood flowing around the gland in a network of fine tubes. The amount of the perspiration is controlled in two ways: by _nerves that regulate the activity of the epithelial_ cells lining the gland, and by nerves that _regulate the size of the blood vessels_ supplying the gland (Fig. 25).
[Illustration: FIG. 24.--PORES on ridges in palm of hand.]
THOUGHT QUESTIONS.--=Freckles, Warts, Moles, Scars, Proud Flesh, Pimples, Blackheads.= Use these names in the proper places below:--
A rough prominence formed by several papillæ growing through the epidermis at a weak spot and enlarging is called a ____. Small patches of pigment developing on the hands and face from much exposure to the sun are called ____. The growth of exposed dermis sprouting through an opening in the epidermis due to accident is called ____. (This should be scraped off and cauterized to aid the epidermis to grow over it again.) Sometimes a cut heals in such a way that no epidermis and therefore no pigment cells cover the place of injury, which is occupied only by white fibrous tissue (cicatricial tissue) of the true skin. In this case the mark left is called a cicatrice or ____. If pores or the openings of oil glands become clogged, but not enlarged, little swellings called ____ may result. An enlarged pore filled with oil and dirt is called a ____. A spot present since birth, dark with pigment, and often containing hairs and blood vessels, is called a ____.
=Regulation of Temperature.=--As is well known, rapid running or violent exercise of any kind causes profuse perspiration. The sweat glands are connected with the brain by means of nerves, and when the body has too much heat, a _nerve impulse from the lowest part of the brain causes the sweat glands to form sweat more rapidly_. Heat and exercise may cause the activity of the sweat glands to increase to forty times the usual rate. The evaporation of the sweat cools the body, for a large amount of heat is required to evaporate a small amount of water (Exp. 4 and 5). This is shown by the cooling effect of sprinkling water on the floor on a warm day. By fanning we hasten the cooling of the body (Exp. 4).
Exercise tends to heat the body, but it also _causes us to breathe faster and causes much blood to flow through the skin_. Both of these effects aid in cooling the body, for the cool air is drawn into the lungs, becomes warm, and takes away heat when it leaves; and the warm blood flowing in the skin loses some of its heat to the cool air in contact with the skin.
=Effects of Alcohol upon the Skin.=--The more blood goes to the skin, the more blood is cooled. The body as a whole may be cooler, but _we feel warmer when there is more blood in the skin because of the effect of the warm blood upon the nerves_ of temperature. There are no nerves for perceiving temperature except in the skin and mucous membrane, and the body has practically no sensation of heat or cold except from the skin or mucous membrane. That alcoholic drinks make the skin red is commonly noticed. Often _the skin is flushed_ by one drink; the bloodshot eyes and purple nose of the toper are the results of habitual use. Can you explain why alcohol brings a _deceptive feeling_ of warmth? Why does alcohol increase the danger of freezing during exposure in very cold weather? During the chill which precedes a fever, the body (except the skin) is really warmer than usual.
Exercise will relieve internal congestion and send the blood to the skin better than alcohol. This is the effect sought by sedentary people who use it to replace exercise. The long and sad experience of the race with alcohol proves that the attempt to adapt the body to its use should be given up.
THOUGHT QUESTIONS. =The Functions of the Skin.=--=1.= State a fact which shows that the skin is a protection; gives off offensive substances; regulates the temperature. =2.= What is lacking in the skin when it cracks or chaps? Why does this occur more often in cold weather? When the hands are bathed with great frequency?
=Effects of Indoor and Outdoor Life.=--_Those who live much out of doors, exposed to sunlight and pure, cold air, are robust and hardy_; while those whose occupations keep them constantly indoors, especially if no physical labor is necessary, show by their pale skins, their fat and flabby, or their thin and emaciated bodies, the weakening effect of such a life. We are descended from ancestors who lived in the open air, and it is impossible for a human being to live much indoors without degeneration of the body and shortening of life.
=A Well-trained Skin.=--We hear a great deal about training the muscles, the brain, the eye, the hand; yet we may fail to realize that the skin also can be trained and its powers developed, or it can be allowed to become weak and powerless. Soundness of the skin is as essential to health as soundness of any other organ. A rosy color indicates good health because of a well-balanced circulation. Paleness often means internal congestion and great liability to indigestion, colds, etc. Hence we think a rosy skin beautiful and a pale skin ugly. With the skin in a healthy condition, the danger of taking most diseases is removed.
=Characteristics of a Vigorous Skin.=--A person who readily takes cold, who is fearful of drafts of air at all times, has a weak skin. To one who has a healthy skin drafts are dangerous only when the skin is moist with perspiration, and the body is inactive; cold drafts may then do harm. Cold air and cold water are the best means of toughening a tender skin. _A bath is to the skin what gymnastic exercises are to the muscles._ The muscle fibers in the walls of the blood vessels and the nerves controlling them need exercise as well as the rest of the body (Fig. 25).
[Illustration: FIG. 25.--BLOOD VESSELS, with the VASO-MOTOR NERVES which accompany and control them.]
=Importance of Bathing.=--_If we followed the outdoor life and wore the scanty clothing of savage races, the rains, the cool air, and the sunlight would keep our skins vigorous and sound._ But want of exercise to induce perspiration allows the sweat glands to become stopped up. The wearing of _clothes_ is a very uncleanly custom. Clothes make the skin inactive, yet confine the impurities which the weakened skin may still be able to excrete. Thick and heavy clothing and overheated rooms prevent the nerves from being stimulated by cold air and sunlight. _The best way to counteract these weakening conditions is by frequent cool or cold baths._ An _air bath_, which consists of exposing the bare skin to the air for half an hour or more before dressing in the morning, may take the place of a cold bath. Even the lower animals bathe: birds, dogs, and many lower animals bathe in the rivers. An elephant sometimes takes a bath by showering water over his back with his trunk.
=Treatment of Burns.=--_Wet_ the burn with a little water and sprinkle common _baking soda_ or flour thickly on it. Bind with a narrow _bandage_. For deeper burns soak a small square of cloth in a strong solution of baking soda, bandage it on wound, and keep it wet with the solution. Olive, cotton seed, and linseed _oils_ are excellent for burns (Exp. 13).
=Hygiene of Bathing.=--A bath should not be taken within an hour after a meal. _Cold baths_ (1) should never be taken in a cold room nor when the skin is cold; (2) are more beneficial in summer and in warm climates, but are necessary in winter for those who live in overheated houses or dress very warmly; (3) should be followed in winter by vigorous rubbing and a glowing reaction; (4) should usually not last longer than one minute in winter. _Warm baths_ (1) are more cleansing than cold baths; (2) should not be used alone but should always be followed by a dash of cold water; (3) are better than cold baths if the body is greatly fatigued; (4) are more beneficial when going to bed than upon rising.
Cold baths and very hot baths are both _stimulants_ to the nervous system and cause an expenditure of nervous energy. For one whose nervous energy is at a very low ebb cold baths may be weakening if prolonged beyond a few seconds. For one with skin relaxed and body sluggish from indoor life, cool baths arouse activity, tone up the body, and may be as beneficial as outdoor exercise in restoring vigorous health. As with every hygienic measure, each person must find out by experience what suits him best.
=Clothing= was first employed for ornament. In cold climates it aids in maintaining the uniform temperature of the body; to it man owes his distinction of being the most widely distributed of animal species. Clothing prevents rapid escape of bodily heat by confining air, a non-conductor of heat, in its meshes. Hence, the effect of clothing varies with the _weave_; likewise with the tendency of its fibers _to keep dry_, for if water replaces air in the meshes, the body loses heat rapidly. For cool clothing the weave should be hard and tight, for warm clothing it should be soft and loose. The warmth of clothing is affected more by its weave than by its weight. The weave may be tested by stretching; the fabric with softest weave will stretch the most (Exp. 8). _Linen_ makes the coolest of all clothing because it weaves hard with small meshes; _silk_ ranks next in coolness. When warmth is desired, linen or _cotton_ garments should be made of fabrics woven like stockings. Linen and cotton both absorb water rapidly and dry rapidly (Exp. 6); if _woolen_ did also, it would make the warmest of all clothing, but it dries so slowly (Exp. 7) that it cools the body after the activity is over instead of drying rapidly and, as with linen and cotton, keeping the body cool during the exertion (Exp. 5). Woolen weaves with the largest air meshes of all materials; hence its warmth increases perspiration, but woolen removes perspiration most slowly and tends to relax the skin if the wearer has an active skin or makes active exertion. Woolen is best for underclothing during extreme cold only or for persons who never make such vigorous muscular exertion as to perspire. In general, cotton or linen is best for underwear. They possess the added advantages of less cost and of not shrinking out of size and shape when washed. A mixture of cotton and silk or of cotton and wool is more durable than either alone. Cotton and linen, unlike woolen, are not attacked by insect pests.
It is better to depend more upon outer clothing than underclothing for warmth. In the Gulf states the wearing of woolen outer clothing indoors during warm weather (which lasts eight months) is unhealthful and uncleanly because of the perspiration absorbed; this is as absurd as to wear cotton outer clothing in Northern states during the eight cold months.
Black clothing absorbs twice, blue almost twice, red and yellow almost one and a half times, as much heat as white clothing (Exp. 10). Which material protects best from radiant heat? (Exp. 9.) Because large blood vessels are near the surface at the _neck, wrists, and ankles_ very thin or no covering at those points aids greatly in keeping the body cool. High collars, long sleeves, and high shoes are unhealthful in warm climates and in summer. What objection to black shoes in summer? Patent leather? Show how women dress more sensibly in hot weather than men.
The =kidneys= are located on each side of the spinal column in the “small of the back” and extend slightly above the level of the waist. They are _bean-shaped organs about four inches long_ (Fig. 26). The kidneys of a sheep or ox closely resemble those of man. They are outside of the peritoneum (Fig. 99) and attached to the rear wall of the abdomen. A large artery (12, colored Fig. 5) goes to each kidney and divides into many capillaries which surround _tubules_ in the kidneys (Fig. 27). The secretion, containing nitrogenous impurities of the blood, is continually being deposited in the tubules, which take it to a _funnel-shaped cavity_ at the inner edge of the kidney (Fig. 26). From this cavity a white tube called the _ureter_ leads down to a storage organ in the pelvis called the bladder.
[Illustration: FIG. 26.--SECTION OF KIDNEY.
_RA_, renal artery; _Py_, pyramids surrounding hollow space from which the ureter (_U_) leads the secretion to the bladder.]
=Changes in Blood in the Kidneys.=--The water holding the nitrogenous waste varies in amount with the amount of water drunk and with the activity of the skin, being less in summer when the perspiration is great. The lungs aid the skin and kidneys in disposing of superfluous moisture. The kidneys have almost the entire responsibility of relieving the body of certain _mineral salts_ and a white crystalline solid called _urea_. This is very injurious if retained, causing headaches, rheumatism, and other troubles.
[Illustration: FIG. 27.--PLAN OF A URINARY TUBULE, _Tb_, with artery _A_, and _V_ in _pV_.]
THOUGHT QUESTIONS. =Hygiene of the Skin.=--=1.= What kind of a scar is not affected by freckles or tan? =2.= Can a scar on a negro be white? =3.= Does a scar on a child grow in size? =4.= Why is heat most oppressive in moist weather? =5.= How do you account for the shape and location of the usual bald spot? =6.= How does the wearing away of the outer cells of the epidermis contribute to the cleanliness of the body? =7.= Why does the palm of the hand absorb water more rapidly than the back of the hand? =8.= Is it more necessary for mental workers to bathe often or change their clothes often? For physical workers? =9.= Is cotton or woolen clothing more liable to stretch or shrink out of shape or size? To catch fire? To make the skin clammy with moisture? To cost more? To be eaten by moths?
[Illustration: FIG. 28.--THE SKELETON.]