Part 10

23. Boys often, though foolishly, thrust a pin into the flesh just above the knee. Why is it not painful?

24. Will ten minutes' practice in a gymnasium answer for a day's exercise?

25. Why would an elastic tendon be unfitted to transmit the motion of a muscle?

26. When one is struck violently on the head, why does he instantly fall?

27. What is the cause of the difference between light and dark meat in a fowl?

III.

THE SKIN.

A protection from the outer world, it is our only means of communicating with it. Insensible itself, it is the organ of touch. It feels the pressure of a hair, yet bears the weight of the body. It yields to every motion of that which it wraps and holds in place. It hides from view the delicate organs within, yet the faintest tint of a thought shines through, while the soul paints upon it, as on a canvas, the richest and rarest of colors.

ANALYSIS OF THE SKIN.

| 1. The Cutis; its Composition and Character. | 1. THE STRUCTURE | 2. The Cuticle; its Composition and Character. | OF THE SKIN. | 3. The Value of the Cuticle. | |4. The Complexion. | | | a. Description. | | b. Method of Growth. | | 1. The Hair.....| c. As an Instrument of | | | Feeling. | 2. THE HAIR AND | | d. Indestructibility of | THE NAILS. | | the Hair. | | | |2. The Nails....| a. Uses. | |b. Method of Growth. | | 3. THE MUCOUS | 1. The Structure. | MEMBRANE | 2. Connective Tissue. | |3. Fat. | | | 1. Number and Kinds of Teeth. | | | | 1. The Two Sets.| 1. The Milk Teeth. | | |2. The Permanent Teeth. | | | 4. THE TEETH. | 2. Structure of the Teeth. | | 3. The Setting of the Tooth in the Jaw. | | 4. The Decay of the Teeth. | |5. The Preservation of the Teeth. | | | 1. The Two Kinds.| 1. Oil Glands. | | |2. Perpiratory Glands. | | | 5. THE GLANDS | 2. The Perspiration. | | 3. The Absorbing Power of the Skin. (See | | Lymphatics.) | | | 1. About Washing and Bathing. | | 2. The Reaction. | | 3. Sea Bathing. | 6. HYGIENE | | a. General Principles. | | | b. Linen. | | | c. Cotton. | |4. Clothing.......| d. Woolen. | | e. Flannel. | | f. Color of Clothing. | | g. Structure of | | Clothing. | | h. Insufficient | | Clothing. | | 1. Erysipelas. | | 2. Salt Rheum. |7. DISEASES. | 3. Corns. | 4. Ingrowing Nails. | 5. Warts. |6. Chilblains.

THE SKIN.

THE SKIN is a tough, thin, close-fitting garment for the protection of the tender flesh. Its perfect elasticity beautifully adapts it to every motion of the body. We shall learn hereafter that it is more than a mere covering, being an active organ, which does its part in the work of keeping in order the house in which we live. It oils itself to preserve its smoothness and delicacy, replaces itself as fast as it wears out, and is at once the perfection of use and beauty.

1. STRUCTURE OF THE SKIN.

CUTIS AND CUTICLE.--What we commonly call the skin--viz., the part raised by a blister--is only the cuticle [Footnote: Cuticula, little skin. It is often styled the scarfskin, and also the epidermis (epi, upon; and derma, skin).] or covering of the cutis or true skin. The latter is full of nerves and blood vessels, while the former neither bleeds [Footnote: We notice this in shaving; for if a razor goes below the cuticle, it is followed by pain and blood. So insensible is this outer layer that we can run a pin through the thick mass at the roots of the nails without discomfort.] nor gives rise to pain, neither suffers from heat nor feels the cold.

The cuticle is composed of small, flat cells or scales. These are constantly shed from the surface in the form of scurf, dandruff, etc., but are as constantly renewed from the cutis [Footnote: We see how rapidly this change goes on by noticing how soon a stain of any kind disappears from the skin. A snake throws off its cuticle entire, and at regular intervals.] below.

Under the microscope, we can see the round cells of the cuticle, and how they are flattened and hardened as they are forced to the surface. The immense number of these cells surpasses comprehension. In one square inch of the cuticle, counting only those in a single layer, there are over a billion horny scales, each complete in itself.--HARTING.

FIG. 22.

VALUE OF THE CUTICLE.--In the palm of the hand, the sole of the foot, and other parts especially liable to injury, the cuticle is very thick. This is a most admirable provision for their protection. [Footnote: We can hold the hand in strong brine with impunity, but the smart will quickly tell us when there is even a scratch in the skin. Vaccine matter must be inserted beneath the cuticle to take effect. This membrane doubtless prevents many poisonous substances from entering the system.] By use, it becomes callous and horny. The boy who goes out barefoot for the first time, "treading as if on eggs," can soon run where he pleases among thistles and over stones. The blacksmith handles hot iron without pain, while the mason lays stones and works in lime, without scratching or corroding his flesh.

THE COMPLEXION.--In the freshly made cells on the lower side of the cuticle, is a pigment composed of tiny grains. [Footnote: These grains are about 1/2000 of an inch in diameter, and, curiously enough, do not appear opaque, but transparent and nearly colorless.--MARSHALL.] In the varying tint of this coloring matter, lies the difference of hue between the blonde and the brunette, the European and the African. In the purest complexion, there is some of this pigment, which, however, disappears as the fresh, round, soft cells next the cutis change into the old, flat, horny scales at the surface.

Scars are white, because this part of the cuticle is not restored. The sun has a powerful effect upon the coloring matter, and so we readily "tan" on exposure to its rays. If the color gathers in spots, it forms freckles. [Footnote: This action of the sun on the pigment of the skin is very marked. Even among the Africans, the skin is observed to lose its intense black color in those who live for many months in the shades of the forest. It is said that Asiatic and African women confined within the walls of the harem, and thus secluded from the sun, are as fair as Europeans. Among the Jews who have settled in Northern Europe, are many of light complexion, while those who live in India are as dark as the Hindoos. Intense heat also increases this coloring matter, and thus a furnace-man's skin, even where protected by clothing, becomes completely bronzed. The black pigment has been known to disappear during severe illness, and a lighter color to be developed in its place. Among the negroes, are sometimes found people who have no complexion, i. e., there is no coloring matter in their skin, hair, or the iris of their eyes. These persons are called Albinos.]

II. HAIR AND NAILS.

The Hair and the Nails are modified forms of the cuticle.

FIG. 23.

THE HAIR is a protection from heat and cold, and shields the head from blows. It is found on nearly all parts of the body, except the palms of the hands and the soles of the feet. The outside of a hair is hard and compact, and consists of a layer of colorless scales, which overlie one another like the shingles of a house; the interior is porous, [Footnote: In order to examine a hair, it should be put on the slide of the microscope, and covered with a thin glass, while a few drops of alcohol are allowed to flow between the cover and the slide. This causes the air, which fills the hair and prevents our seeing its structure, to escape.] and probably conveys the liquids by which it is nourished.

Each hair grows from a tiny bulb (papilla), which is an elevation of the cutis at the bottom of a little hollow in the skin. From the surface of this bulb, the hair is produced, like the cuticle, by the constant formation of new cells at the bottom. When the hair is pulled out, this bulb, if uninjured, will produce a new one; but, when once destroyed, it will never grow again. [Footnote: Hair grows at the rate of about five to seven inches in a year. It is said to grow after death. This appearance is due to the fact that by the shrinking of the skin the part below the surface is caused to project, which is especially noticeable in the beard.] The hair has been known to whiten in a single night by fear, fright, or nervous excitement. When the color has once changed, it can not be restored. [Footnote: Hair dyes, or so-called "hair restorers," are almost invariably deleterious substances, depending for their coloring properties upon the action of lead or lunar caustic. Frequent instances of hair poisoning have occurred, owing to the common use of such dangerous articles. If the growth of the hair be impaired, the general constitution or the skin needs treatment. This is the work of a skillful physician, and not of a patent remedy. Dame Fashion has her repentant freaks as well as her ruinous follies, and it is a healthful sign that the era of universal hair dyeing has been blotted out from her present calendar, and the gray hairs of age are now honored with the highest place in "style" as well as in good sense and cleanliness.] (See p. 285.)

Wherever hair exists, tiny muscles are found, interlaced among the fibers of the skin. These, when contracting under the influence of cold or electricity, pucker up the skin, and cause the hair to stand on end. [Footnote: In horses and other animals which are able to shake the whole skin, this muscular tissue is much more fully developed than in man.] The hairs themselves are destitute of feeling. Nerves, however, are found in the hollows in which the hair is rooted, and so one feels pain when it is pulled. [Footnote: These nerves are especially abundant in the whiskers of the cat, which are used as feelers.] Thus the insensible hairs become wonderfully delicate instruments to convey an impression of even the slightest touch.

FIG. 24.

Next to the teeth and bones, the hair is the least destructible part of the body, and its color is often preserved for many years after the other portions have gone to decay. [Footnote: Fine downy hairs, such as are general upon the body, have been detected in the little fragments of skin found beneath the heads of the nails by which, centuries ago, certain robbers were fastened to the church doors, as a punishment for their sacrilege.]