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air tubes. It is pink from the network of capillaries, through which the blood flows. Our lungs are much

like the chick

en’s, except that Lungs

they are larger (Fig. 278).

Each lung is surrounded by a sac of connective tissue to protect it. If the sac becomes inflamed,

the disease is Fig. 278. — Lungs and air sacs of a bird.

called pleurisy (plūr'i-sì). The heart lies between the two lung sacs (see § 378). The lungs, as we should expect, are elastic, like a sponge. While they are compressed when we exhale air, they expand rapidly when we inhale it.

Air Sacs

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The lungs are the meeting place of air and blood. The total surface of their air sacs and tubes is probably 2000 square feet in a grown person, so that a great deal of blood is furnished with oxygen at one time. A room 20 feet wide would need to be how many feet long to have a floor area of 2000 square feet ?

Did you know that the breathing organs of flying birds are even larger in proportion than our own? The birds have air sacs in various parts of the body, and even inside of some of the hollow bones. How does this help the exchange of oxygen and carbon dioxide? How do the hollow bones, filled with air, affect the density of the bird's body? Such birds breathe much more rapidly than we do, and their heart beats more rapidly, too. Their body temperature may reach 110° F. What is ours?

388. What is the Voice ? — We have learned that the boxlike organ, through which the air passes in going from the throat to the windpipe, is called the larynx. It contains the organs of the voice, the vocal cords (see § 265). These cords are two strips of tough connective tissue (Fig. 279), stretched between the cartilage of the larynx. They leave a V-shaped opening between them when they Fig. 279. – The vocal cords in two positions : for

producing a sound, and for quiet breathing. are at rest, but they have muscles, which draw them closer together, and stretch them, when we speak or sing. Air, passing these tight cords as it goes through the larynx, makes them vibrate like tuning forks or other sounding bodies (see § 265). The vibrations produce sounds. The loudness of the sound depends upon the amount of air you force past the vocal cords. When you shout, what kind of breath do you take?

[graphic]

When Producing Sound

During Quiet Breathing

The highness, or lowness (pitch), of a vocal sound depends upon the length and thickness of the cords. The more tightly the cords are drawn, the shorter they become. Short cords can vibrate a greater number of times per minute than the long cords. Thus, when the cords are short, the pitch is high. When the cords are long, the pitch is low. As a man has a larger larynx than a woman, his cords are longer and larger than hers. Therefore he cannot make his cords as short as she can hers. As a result, he has a lower voice than a woman has (see § 268).

The quality of a voice, which enables us to distinguish one person

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from another, is due to the peculiar way in which each person uses his air passages, as well as the shape of the air passages themselves (see § 269). We form the sounds of letters by changing the shape or position of the mouth, tongue, and lips. Vowels (a, e, i, o, u, and y) are the sounds which are more purely vocal, or voice, sounds, and are less changed by the lips and mouth than the consonants.

When we whisper, the vocal cords are drawn together as for sounds made with the voice, but not taut enough to vibrate and produce voice.

389. Do the Organs of Respiration Need Care ? We have already learned (see $$ 52 and 54) some of the things we should do, and should not do, if we wish to keep our breathing organs in good condition. The harmful germs of the air reach us chiefly through the breathing organs. If these organs are in good health, so that fresh air can reach every part of them, they are usually able to protect themselves and the body ; but if they are not strong, or are weakened by neglect and abuse, serious diseases may attack us. When you recall (see $ 204) how easily molds and bacteria multiply in warm, moist food, you will understand that the breathing passages and lungs, which are also warm and moist, may be good breeding places for germs of certain diseases. We should see to it that we always have fresh, clean, moist air to breathe, and that the lungs get sufficient exercise.

If we ought to breathe air free from dust, and containing as little as possible of gases that are not in pure air, what do you think of the practice of smoking tobacco? Will the inhaled smoke and the gases produced by the slowly burning tobacco be likely to help, or injure, the delicate tissues of the throat and lungs ?

Physicians tell us that the habit of drinking alcoholic liquors weakens the lungs; so that the drinker is more likely to catch lung diseases than one who abstains from liquor.

What do you think of the practice of wearing tight clothing and a tight belt? Do you think that a person can breathe correctly, if the abdomen is compressed, and its organs, and those of the chest, are forced out of place?

Do you think that a cold is only a harmless discomfort, and does not need any special attention? We must remember that colds are dangerous, because they lower the power of the throat and lungs to resist disease, and make it easy for harmful germs to attack us. Breathing deeply of fresh air, if we have been chilled, will often bring a warm glow to the skin, and work off a cold.

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390. What is the Structure of the Skin ? — Does the skin form a part of our respiratory system? Review § 385 for the action of the skin in the lowest animals. Yet even in our own bodies, the skin assists the lungs and kidneys in getting rid of the waste of the cells (see § 383), and it is the regulator, along with the lungs and blood, of the body's temperature (see $$ 71, 374, and 392).

Review $ 341 for the description of an animal skin.

Our own skin has a similar struc- Fig. 280. — The structure of the skin. Note

its three layers, especially the complex nature ture (Fig. 280). The of the dermis, or true skin.

[graphic]

outside layer is the epidermis, or cuticle (kūt'i-kl). This has neither nerves nor blood vessels, but protects the true skin, or dermis, which is beneath it. The outer surface of the epidermis is being worn off constantly (why?); but its lower cells are in contact with lymph, and so get food for forming new cells. Where the skin is put to rough use, as in the palms of the hands, a thick layer of epidermis, called a callus, is formed.

The dermis is thicker than the epidermis, and is made up of strong fibers of connective tissue, together with blood vessels and nerves. On its surface are little elevations, some of which contain the bodies that bring about our sense of touch and of temperature (see § 404).

The color of the skin is due partly to the blood, which shows through the capillaries, and partly to the coloring matter in the lower layers of the cells of the epidermis. Tan and freckles are caused by larger amounts of coloring matter, formed by the action of sun and wind (see § 240). A layer of fat is found under the dermis.

In the lower part of the dermis are the sweat glands. These are partly coiled tubes, which take from the blood the liquid we call perspiration, and then pour it out upon the skin (see $ 71). The sweat glands are under the control of our nervous system (see Chapter XXXIX), and pour out only a little perspiration when we are cool, but a great deal when the body is hot and needs to be cooled.

391. How Do the Hair and Nails Grow? — Do the hair and nails look like skin? Although they differ from skin in appearance, they are really special forms of the epidermis, produced to protect the skin (Fig. 281).

A hair is a very small tube which grows in a pocket of the dermis. The part projecting from the skin, like the epidermis itself, has neither blood nor nerves.

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