AIR-TUBES AND AIR-CELLS.AIR-TUBES AND AIR-CELLS.
This is a little box made of cartilage, called the voice-box, because by means of this curious little apparatus we are able to talk and sing. Two little white bands are stretched across the inside of the voice-box. When we speak, these bands vibrate just as do the strings of the piano. These bands are called thevocal cords.
16. The Epiglottis.—At the top of the voice-box is placed a curious trap-door which can be shut down so as to close the entrance to the air-passages of the lungs. This little door has a name rather hard to remember. It is called theepiglottis(ep-i-glot'-tis). The cover of the voice-box closes whenever we swallow anything. This keeps food or liquids from entering the air passages. If we eat or drink too fast the voice-box will not have time to close its little door and prevent our being choked. Persons have been choked to death by trying to swallow their food too fast. Do you not think this is a very wonderful door that can open and shut just when it should do so without our thinking anything about it?
17. The Nostrils and the Soft Palate.—The air finds its way to the lungs through the mouth or through the two openings in the nose called thenostrils. From each nostril, three small passages lead backward through the nose. At the back part of the nasal cavity the passages of the two sides of the nose come together in an open space, just behind the soft curtain which hangs down at the back part of the mouth. This curtain is called thesoft palate. Through the opening behind this curtain the air passes down into the voice-box and then into the lungs.
18. The Pleura.—In the chest the air tubes and lung of each side are enclosed in a very thin covering, called thepleura. The cavity of the chest in which the lungs are suspended is also lined by the pleura. A limpid fluid exudes from the pleura which keeps it moist, so that when the two surfaces rub together, as the lungs move, they do not become chafed and irritated.
19. Walls of the Chest.—The ribs form a part of the framework of the chest. The ribs are elastic. The spaces between them are filled up with muscles, some of which draw the ribs together, while others draw them apart. Can you tell any reason why the walls of the chest are elastic? The lower wall or floor of the chest cavity is formed by a muscle called thediaphragm, which divides the trunk into two cavities, the chest and the abdomen.
20. How we Use the Lungs.—Now let us notice how we use the lungs and what takes place in them. When we use a pair of bellows, we take hold of the handles and draw them apart. The sides of the bellows are drawn apart so that there is more room between the sides. The air then rushes in to fill the space. When the bellows are full, we press the handles together and the air is forced out.
21.It is in just this way that we breathe.When we are about to take a long breath, the muscles pull upon the sides of the chest in such a way as to draw them apart. At the same time the diaphragm draws itself downward. By these means, the cavity of the chest is made larger and air rushes in through the nose or mouth to fill the space. When the muscles stop pulling, the walls of the chest fall back again to their usual position and the diaphragm rises. The cavity of the chest then becomes smaller and the air is forced out through the nose or mouth. This process is repeated every time we breathe.
22.We breathe once for each four heart-beats. Small children breathe more rapidly than grown persons. We usually breathe about eighteen or twenty times in a minute.
23. How Much the Lungs Hold.—Every time we breathe, we take into our lungs about two thirds of a pint of air and breathe out the same quantity. Our lungs hold, however, very much more than this amount. A man, after he has taken a full breath, can breathe out a gallon of air, or more than ten times the usual amount. After he has breathed out all he can, there is still almost half a gallon of air in his lungs which he cannot breathe out. So you see the lungs hold almost a gallon and a half of air.
24.Do you think you can tell why Nature has given us so much more room in the lungs than we ordinarily use in breathing? If you will run up and down stairs three or four times you will see why we need this extra lung-room. It is because when we exercise vigorously the heart works very much faster and beats harder, and we must breathe much faster and fuller to enable the lungs to purify the blood as fast as the heart pumps it into them.
25. The Two Breaths.—We have learned that the air which we breathe out contains something which is not found in the air which we breathe in. This is carbonic-acid gas. How many of you remember how we found this out? We can also tell this in another way. If we put a candle down in a wide jar it will burn for some time. If we breathe into the jar first, however, the candle will go out as soon as we put it into the jar. This shows that the air which we breathe out contains something which will put a candle out. This is carbonic-acid gas, which is a poison and will destroy life.
26. Other Poisons.—The air which we breathe out also contains other invisible poisons which are very much worse than the carbonic-acid gas. These poisons make the air of a crowded or unventilated room smell very unpleasant to onewho has just come in from the fresh air. Such air is unfit to breathe.
27. The Lungs Purify the Blood.—We have learned that the blood becomes dark in its journey through the body. This is because it loses its oxygen and receives carbonic-acid gas. While passing through the capillaries of the lungs, the blood gives out the carbonic-acid gas which it has gathered up in the tissues, and takes up a new supply of oxygen, which restores its scarlet hue.
28. How the Air is Purified.—Perhaps it occurs to you that with so many people and animals breathing all the while, the air would after a time become so filled with carbonic-acid gas that it would be unfit to breathe. This is prevented by a wonderful arrangement of Nature. The carbonic-acid gas which is so poisonous to us is one of the most necessary foods for plants. Plants take in carbonic-acid gas through their leaves, and send the oxygen back into the air ready for us to use again.
29.We have already learned that the oxygen taken in by the lungs is carried to the various parts of the body by the little blood corpuscles. The effect of strong liquors is to injure these corpuscles so that they cannot carry so much oxygen as they ought to do. For this reason, the blood of a drunkard is darker in color thanthat of a temperate person, and contains more carbonic-acid gas. The drunkard's lungs may supply all the air he needs, but his blood has been so damaged that he cannot use it. Excessive smoking has a similar effect.
1. Our bodies need air, just as a candle or a fire does.
2. A small animal shut up in a close jar soon dies for want of air. We need the oxygen which the air contains.
3. Oxygen causes a sort of burning in our bodies.
4. The burning in our bodies keeps us warm, and destroys some of the waste matters.
5. The breathing organs are the windpipe and bronchial tubes, the voice-box, the epiglottis, the nostrils, the soft palate, the lungs, the air-cells, the pleura, the diaphragm, and the chest walls.
6. When we breathe we use our lungs like a pair of bellows.
7. A man's lungs hold nearly one and a half gallons of air.
8. In ordinary breathing we use less than a pint of air, but when necessary we can use much more.
9. The air we breathe out contains carbonic-acid gas and another invisible poison.
10. A candle will not burn in air which has been breathed, and animals die when confined in such air.
11. The lungs purify the blood. While passing through the lungs, the color of the blood changes from purple to bright red.
12. Plants purify the air by removing the carbonic-acid gas.
13. Alcohol and tobacco injure the blood corpuscles so that they cannot take up the oxygen from the air which the lungs receive.
1. Pure Air Necessary.—A person may go without eating for a month, or without drinking for several days, and still live; but a strong man will die in a few moments if deprived of air. It is very important that we breathe plenty of pure air. There are many ways in which the air becomes impure.
2. Bad Odors.—Anything which rots or decays will in so doing produce an unpleasant odor. Bad odors produced in this way are very harmful and likely to make us sick. Many people have rotting potatoes and other vegetables in their cellars, and swill barrels, and heaps of refuse in their back yards. These are all dangerous to health, and often give rise to very serious disease. We should always remember that bad odors caused by decaying substances are signs of danger to health and life, and that these substances should be removed from us, or we should get away from them, as soon as possible.
3. Germs.—The chief reason why bad odorsare dangerous is that they almost always have with them little living things calledgerms. Germs are so small that they cannot be seen by the naked eye: it takes a strong microscope to enable us to see them, but they are so powerful to do harm that if we receive them into our bodies they are likely to make us very sick, and they often cause death.
4. Contagious Diseases.—You have heard about diphtheria and scarlet fever and measles, and other "catching diseases." When a person is sick with one of these diseases, the air about him is poisoned with germs or something similar, which may give the same disease to other persons who inhale it. So when a person is sick from one of these diseases, it is very important that he should be put in a room by himself and shut away from every one but the doctor and the nurse. It is also necessary that all the clothing and bedding used by the sick person, and everything in the room, as well as the room itself, should be carefully cleansed and disinfected when the person has recovered, so as to wipe out every trace of the disease. The writer has known many cases in which persons who have been sick with some of these diseases were careless and gave the disease to others who died of it, although they themselves recovered. Do younot think it very wrong for a person to give to another through carelessness a disease which may cause his death?
5.Unhealthful vapors and odors of various sorts arise from cisterns and damp, close places under a house. Rooms which are shaded and shut up so closely that fresh air and sunshine seldom get into them should be avoided as dangerous to health.
6. Breath-Poisoned Air.—The most dangerous of all the poisons to which we are exposed through the air are those of the breath, of which we learned in a preceding lesson. We need plenty of fresh air to take the place of the air which we poison by our breath. Every time we breathe, we spoil at leasthalf a barrelful of air. We breathe twenty times a minute, and hence spoil ten barrels of air in one minute. How many barrels would this make in one hour? We need an equal quantity of pure air to take the place of the spoiled air, or not less than ten barrels every minute, orsix hundred barrels every hour.
7. Ventilation.—The only way to obtain the amount of fresh air needed, when we are shut up in-doors, is to have some means provided by which the fresh air shall be brought in and the old and impure air carried out. Changing the air by such means is calledventilation.Every house, and especially every sleeping-room, should be well ventilated. School-houses, churches, and other places where many people gather, need perfect ventilation. Ask your teacher to show you how the school-room is ventilated; and when you go home, talk to your parents about the ventilation of the house in which you live.
8.Many people ventilate their houses by opening the doors and windows. This is a very good way of ventilating a house in warm weather, but is a very poor way in cold weather, as it causes cold draughts, and makes the floor cold, so that it is difficult to keep the feet warm. It is much better to have the air warmed by a furnace or some similar means, before it enters the rooms. There ought also to be in each room a register to take the foul air out, so that it will not be necessary to open the windows. This register should be placed at the floor, because when the pure air enters the room warm, it first rises to the upper part of the room, and then as it cools and at the same time becomes impure, it settles to the floor, where it should be taken out by the register.
9. How to Breathe.—We should always take pains to expand the lungs well in breathing, and to use the entire chest, both the upper and the lower part. Clothing should be worn in sucha way that every portion of the chest can be expanded. For this reason it is very wrong to wear the clothing tight about the waist. Clothing so worn is likely to cause the lungs to become diseased.
10. Bad Habits.—Students are very apt to make themselves flat-chested and round-shouldered by leaning over their desks while writing or studying. This is very harmful. We should always use great care to sit erect and to draw the shoulders well back. Then, if we take pains to fill the lungs well a great many times every day, we shall form the habit of expanding the lungs, and shall breathe deeper, even when we are not thinking about doing so.
11. Breathing through the Nose.—In breathing, we should always take care to draw the air in through the nose, and not through the mouth. The nose acts as a strainer, to remove particles of dust which might do harm if allowed to enter the lungs. It also warms and moistens the air in cold weather. The habit of breathing through the mouth often gives rise to serious disease of the throat and lungs.
12. Effects of Alcohol and Tobacco upon the Lungs.—Both alcohol and tobacco produce disease of the breathing organs. Smoking injures the throat and sometimes causes loss ofsmell. Serious and even fatal diseases of the lungs are often caused by alcohol.
13.Many people suppose that the use of alcohol will save a man from consumption. This is not true. A man may become a drunkard by the use of alcohol, and yet he is more likely to have consumption than he would have been if he had been a total abstainer. "Drunkard's consumption" is one of the most dreadful forms of this disease.
1. Pure air is as necessary as food and drink.
2. Anything which is rotting or undergoing decay causes a bad odor, and thus makes the air impure.
3. Foul air contains germs which cause disease and often death.
4. Persons sick with "catching" diseases should be carefully avoided. Such persons should be shut away from those who are well, and their rooms and clothing should be carefully cleansed and disinfected.
5. The breath poisons the air about us. Each breath spoils half a barrelful of air.
6. We should change the air in our houses, or ventilate them, so that we may always have pure air.
7. We should always keep the body erect, and expand the lungs well in breathing.
8. The clothing about the chest and waist should be loose, so that the lungs may have room to expand.
9. Always breathe through the nose.
10. Tobacco causes disease of the throat and nose.
11. Alcohol causes consumption and other diseases of the lungs.
1. The Skin.—The skin is the covering of the body. It fits so exactly that it has the precise shape of the body, like a closely fitting garment. If you will take up a little fold of the skin you will see that it can be stretched like a piece of india-rubber. Like rubber, when it is released it quickly contracts and appears as before.
2. The Bark of Trees.—Did you ever peel the bark off of a young tree? If so, you have noticed that there were really two barks, an outer bark, as thin as paper, through which you could almost see, and an inner and much thicker bark, which lay next to the wood of the tree. You can peel the outer bark off without doing the tree much harm. Indeed, if you will notice some of the fruit or shade trees in the yard, at home, you will see that the outer bark of the tree peels itself off, a little at a time, and that new bark grows in its place. If you tear off the inner bark, however, it will injure the tree. It will make it bleed, or cause the sap to run.The sap is the blood of the tree. The bark is the skin of the tree. When the bare place heals over, an ugly scar will be left.
3. The Cuticle.—Our bodies, like trees, have two skins, or really one skin with an outer and an inner layer. When a person burns himself so as to make a blister, the outer skin, called thecuticle, is separated from the inner by a quantity of water or serum poured out from the blood. This causes the blister to rise above the surrounding skin. If you puncture the blister the water runs out. Now we may easily remove the cuticle and examine it. The cuticle, we shall find, looks very much like the skin which lines the inside of an egg-shell, and it is almost as thin.
4.The cuticle is very thin in most parts of the body, but in some places, as the palms of the hands and the soles of the feet, it is quite thick. This is because these parts of the skin come in contact with objects in such a way as to be liable to injury if not thus protected. The cuticle has no blood-vessels and very few nerves. With a fine needle and thread you can easily take a stitch in it without making it bleed or causing any pain.
5. The Pigment.—The under side of the cuticle is colored by little particles of pigment or coloring matter. The color of this pigment differs in different races. In the negro, the color of the pigment is black. In some races the pigment is brown. In white persons there is very little pigment, and in some persons, called albinos, there is none at all.
6. The Inner or True Skin.—The inner skin, like the inner bark of a tree, is much thicker than the outer skin. It is much more important, and for this reason is sometimes called thetrue skin. It contains nerves and blood-vessels.
SKIN OF PALM OF HAND MAGNIFIED.SKIN OF PALM OFHAND MAGNIFIED.
7. The Sweat Glands.—If you look at the palm of the hand you will see many coarse lines, and by looking much closer you will see that the palm is completely covered with very fine ridges and furrows. Now, if you examine these ridges with a magnifying-glass, you will find arranged along each ridge a number of little dark spots. Each of these points is the mouth of a very small tube. This is called asweat duct. These ducts run down through both the outer and inner layers of the skin. At the under side of the true skin the end of the tube is rolled up in a coil, as you can see by looking at the illustration on the following page. The coiled parts of the tubes are calledsweat glands, because they separate from the blood the fluid which we call sweat or perspiration.
8. The Oil Glands.—There are other little glands in the skin which make fat or oil. The oil is poured out upon the skin to keep it soft and smooth.
THE STRUCTURE OF THE SKIN.THE STRUCTURE OF THE SKIN.
9. The Hair.—There are some curious little pockets in the skin. Out of each of these pockets grows a hair. On some parts of the body the hairs are coarse and long; on other parts they are fine and short.
10.Many of the ducts leading from the oil glands open into the pockets or pouches from which the hairs grow. The oil makes the hair soft and glossy. Nature has thus provided an excellent means for oiling the hair.
11.The hair is chiefly useful as a protection. It is also an ornament.
12. The Nails.—The nails of the fingers and the toes grow out of little pockets in the skin just as the hairs do. Both the hair and the nails are really parts of the outer skin, which is curiously changed and hardened. The nails lie upon the surface of the true skin and grow from the under side as well as from the little fold of skin at the root of the nail. They are made to give firmness and protection to the ends of the fingers and toes. The nails of the fingers are also useful in picking up small objects and in many other ways.
13. Uses of the Skin.—The skin is useful in several ways:
(1)It Removes Waste.—The sweat glands and ducts are constantly at work removing from the blood particles which have been worn out and can be of no further use. If we get verywarm, or if we run or work very hard, the skin becomes wet with sweat. In a little while, if we stop to rest, the sweat is all gone. What becomes of it? You say it dries up, which means that it has passed off into the air. Sweating is going on all the time, but we do not sweat so much when we are quiet and are not too warm, and so the sweat dries up as fast as it is produced, and we do not see it. Nearly a quart of sweat escapes from the skin daily.
(2)Breathing through the Skin.—We breathe to a slight extent through the skin. There are some lower animals which breathe with their skins altogether. A frog can breathe with its skin so well that it can live for some time after its lungs have been removed. Breathing is an important part of the work of the skin, and we should be careful, by keeping it clean and healthy, to give it a good chance to breathe all that it can.
(3)The Skin Absorbs.—The skin absorbs many substances which come in contact with it, and hence should be kept clean. If the foul substances which are removed in the sweat are allowed to remain upon the skin, they may be taken back into the system and thus do much harm.
(4)The Skin has Feeling.—When anythingtouches the skin we know it by the feeling. We can tell a great many things about objects by feeling of them. If we happen to stick a pin into the skin we feel pain. We are also able to tell the difference between things which are hot and those which are cold. Thus the sense of feeling which the skin has is very useful to us.
(5)The Skin Protects the Body.—The skin is a natural clothing which protects us much better than any other kind of clothing could. It is so soft and pliable that it cannot hurt the most delicate part which it covers, yet it is very strong and tough.
1. The skin is the covering of the body. It has two layers, the outer, called the cuticle, and the inner, called the true skin.
2. A substance called pigment is found between the two skins. This gives the skin its color.
3. The true skin has blood-vessels and nerves, but the cuticle has no blood-vessels and very few nerves.
4. In the true skin are glands which produce sweat, and others which make fat, or oil.
5. The nails are really a part of the skin. They are firm and hard, and protect the ends of the fingers and the toes.
6. The hair grows from the true skin. The hair is made soft and glossy by oil from the oil glands of the skin.
7. The skin is a very useful organ. It removes waste matters, it breathes, it absorbs, it has feeling, and it protects the body.
1. Uses of the Pores of the Skin.—Many years ago, at a great celebration, a little boy was covered all over with varnish and gold leaf, so as to make him represent an angel. The little gilded boy looked very pretty for a short time, but soon he became very sick, and in a few hours he was dead. Can you guess what made him die? He died because the pores of his skin were stopped up, and the sweat glands could not carry off the poisonous matter from his body.
2. Cleanliness.—Did you ever know of a boy who had his skin varnished? Not exactly, perhaps; but there are many boys who do not have their skins washed as often as they ought to be, and the sweat and oil and dead scales form a sort of varnish which stops up the little ducts and prevents the air from getting to the skin, almost as much as a coat of varnish would do.
3. The Sweat Glands.—The sweat glands and ducts are like little sewers, made to carry away some of the impurities of the body. There areso many of them that, if they were all put together, they would make a tube two or three miles long. These little sewers drain off almost a quart of impurities in the form of sweat every day. So you see that it is very important for the skin to be kept clean and healthy.
4. Bathing.—A bird takes a bath every day. Dogs and many other animals like to go into the water to bathe. Some of you have seen a great elephant take a bath by showering the water over himself with his trunk. To keep the skin healthy we should bathe frequently.
5.When we take a bath for cleanliness it is necessary to use a little soap, so as to remove the oil which is mixed up with the dry sweat, dead scales, and dirt which may have become attached to the skin.
6.It is not well to take hot baths very often, as they have a tendency to make the skin too sensitive. Bathing in cool water hardens the skin, and renders one less likely to take cold.
7. The Clothing.—The skin should be protected by proper clothing, but it is not well to wear more than is necessary, as it makes the skin so sensitive that one is liable to take cold.
8. The Proper Temperature of Rooms.—It is also very unwise for a person to keep the rooms in which he lives too warm, and to staytoo much in-doors, as it makes him very liable to take cold when he goes out-of-doors. One who is out of doors in all kinds of weather seldom takes cold.
9. Care of the Hair and the Nails.—The scalp should be kept clean by thorough and frequent washing and daily brushing. Hair oils are seldom needed. If the skin of the head is kept in a healthy condition, the hair requires no oil.
10.The habit of biting and picking the fingernails is a very unpleasant one, and keeps the nails in a broken and unhealthy condition. The nails should be carefully trimmed with a sharp knife or a pair of scissors.
11. Effects of Narcotics and Stimulants upon the Skin.—Alcohol, tobacco, opium, and all other narcotics and stimulants have a bad effect upon the skin. Alcohol often causes the skin to become red and blotched, and tobacco gives it a dingy and unhealthy appearance.
1. If the pores of the skin are closed, a person will die.
2. We should bathe often enough to keep the skin clean.
3. We should not keep our rooms too warm, and should avoid wearing too much clothing.
4. Alcohol, tobacco, and other stimulants and narcotics injure the skin.
1. The Kidneys.—The kidneys are among the most important organs of the body. They are in the cavity of the abdomen, near the back-bone, up under the lower border of the ribs. Perhaps you have seen the kidneys of a sheep or a hog. If you have, you know very nearly how the kidneys of our own bodies appear.
KIDNEY.KIDNEY.
2. The Work of the Kidneys.—The work of the kidneys is to separate from the blood certain very poisonous substances, which would soon cause our death if they were not removed. It is very important to keep these useful organs in good health, because a person is certain to die very soon when the kidneys are in any way seriously injured.
3. How to Keep the Kidneys Healthy.—One way of keeping the kidneys in good health is todrink plenty of pure water, and to avoid eating too much meat and rich food. Pepper, mustard, and other hot sauces are very harmful to the kidneys.
4. Importance of Keeping the Skin Clean.—The work of the kidneys is very similar to that of the skin; and when the skin does not do its full duty, the kidneys have to do more than they should, and hence are likely to become diseased. For this reason, persons who allow their skins to become inactive by neglecting to bathe frequently are apt to have disease of the kidneys.
5. Effects of Alcohol and Tobacco upon the Kidneys.—A piece of beef placed in alcohol soon becomes dry and hard, and shrivels up as though it had been burned. The effect upon the kidneys of drinking strong liquor is almost the same. Beer and hard cider also do the kidneys harm, sometimes producing incurable disease of these important organs.
1. The kidneys somewhat resemble the skin in their structure and in their work.
2. The kidneys remove from the blood some poisonous substances.
3. To keep the kidneys healthy we should drink plenty of water, avoid irritating foods and drinks, and keep the skin in health by proper bathing.
4. The drinking of strong liquors often causes incurable disease of the kidneys.
1. The Bones.—In an earlier chapter we learned something about the bones. This we must try to recall. You will remember that we called the bones the framework of the body, just as the timbers which are first put up in building a house are called its frame.
2. The Skeleton.—All the bones together make up theskeleton. (See page 95.) There are about two hundred bones in all. They are of many different shapes. They vary in size from the little bones of the ear, which are the smallest, to the upper bone of the leg, which is the largest in the body.
3.The skeleton is divided into four parts: theskull, thetrunk, thearms, and thelegs. We must learn something more about the bones of each part.
4. The Skull.—Theskullis somewhat like a shell. It is made of a number of bones joined together in such a way as to leave a hollow place inside to hold the brain. The front partof the skull forms the framework of the face and the jaws. In each ear there are three curious little bones, which aid us in hearing.
5. The Trunk.—The bones of the trunk are, theribs, thebreast-bone, thepelvis, and theback-bone. The bones of the trunk form a framework to support and protect the various organs within its cavities.
6. The Ribs.—There are twelveribson each side. The ribs join the back-bone at the back. They are connected by cartilage to the breast-bone in front. They look somewhat like the hoops of a barrel. With the breast-bone and the back-bone they form a bony cage to contain and protect the heart and the lungs.
7. The Pelvis.—The pelvis is at the lower part of the trunk. It is formed by three bones, closely joined together. The large bones at either side are called the hip-bones. Each hip-bone contains a deep round cavity in which the upper end of the thigh-bone rests.
8. The Back-bone.—Theback-bone, or spinal column, is made up of twenty-four small bones, joined together in such a way that the whole can be bent in various directions. The skull rests upon the upper end of the spinal column. The lower end of the back-bone forms a part of the pelvis.
SKELETON OF A MAN.SKELETON OF A MAN.
9. The Spinal Canal.—Each of the separate bones that make up the back-bone has an opening through it, and the bones are so arranged, one above another, that the openings make a sort of canal in the back-bone. By the connection of the spinal column to the head, this canal opens into the cavity of the skull. Through this canal there passes a peculiar substance called thespinal cord, of which we shall learn more at another time.
10. The Arms.—Each of the arms has five bones, besides the small bones of the hand. They are thecollar-bone, which connects the shoulder to the breast-bone, theshoulder-blade, at the back of the shoulders, theupper arm-bone, between the shoulder and the elbow, and the twolower arm-bones, between the elbow and the wrist. There are eight little bones in the wrist, five in that part of the hand next to the wrist, and fourteen in the fingers and thumb.
11. The Legs.—The bones of the leg are thethighorupper leg-bone, theknee-panorknee-cap, which covers the front of the knee, the two bones of thelower leg, theheel-boneand six other bones in theankle, five bones in that part of the foot next to the ankle, and fourteen bones in thetoes.
12. Use of the Bones.—The skeleton is not only necessary as a framework for the body,but it is useful in other ways. Some of the bones, as the skull, protect delicate parts. The brain is so soft and delicate that it would be very unsafe without its solid bony covering. The spinal cord also needs the protection which it finds in the strong but flexible back-bone. The bones help to move our hands and arms, and assist us in walking.
13. The Joints.—The places where two or more bones are fastened together are calledjoints. Some joints we can move very freely, as those of the shoulder and the hip. Others have no motion at all, as those of the bones of the skull.
14. Cartilage.—The ends of bones which come together to form a joint are covered with a smooth, tough substance, which protects the bone from wear. This is calledgristleorcartilage. You have, no doubt, seen the gristle on the end of a "soup-bone" or on one of the bones of a "joint of beef."
15.The joint contains a fluid to oil it, so that the ends of the bones move upon each other very easily. If the joints were dry, every movement of the body would be very difficult and painful.
16.The bones are held together at the joints by means of strong bands calledligaments.
17. How the Bones are Made.—The bonesare not so solid as they seem to be. The outside of most bones is much harder and firmer than the inside. Long bones, like those of the arms and the legs, are hollow. The hollow space is filled withmarrow, in which are the blood-vessels which nourish the bone.
18. An Experiment.—If you will weigh a piece of bone, then burn it in the fire for several hours, and then weigh it again, you will find that it has lost about one third of its weight. You will also notice that it has become brittle, and that it seems like chalk.
19. Why the Bones are Brittle.—The hard, brittle portion of a bone which is left after it has been burned contains a good deal of chalk and other earthy substances, sometimes called bone-earth. It is this which makes the bones so hard and firm that they do not bend by the weight of the body. When we are young, the bones have less of this bone-earth, and so they bend easily, and readily get out of shape. When we get old, they contain so much bone-earth that they become more brittle, and often break very easily.
20.A person's height depends upon the length of his bones. The use of alcohol and tobacco by a growing boy has a tendency to stunt the growth of his bones, so that they do not develop as they should.
4. The bones are useful for support, protection, and motion.
5. The place where two bones join is called a joint.
6. The tough substance which covers the ends of many bones is called cartilage or gristle.
7. The joints are enabled to work easily by the aid of a fluid secreted for that purpose.
8. The ends of the bones are held together in a joint by means of ligaments.
9. Bones are about two thirds earthy matter and one third animal matter.
10. The use of alcohol and tobacco may prevent proper development of the bones.
1. Composition of the Bones.—Our bones, like the rest of our bodies, are made of what we eat. If our food does not contain enough of the substances which are needed to make healthy bone, the bones will become unhealthy. They may be too soft and become bent or otherwise misshapen. This is one of the reasons why bread made from the whole grain is so much more healthful than that made from very fine white flour. In making fine white flour the miller takes out the very best part of the grain, just what is needed to make strong and healthy bones. Oatmeal is a very good food for making healthy bones.
2. Bones of Children.—Sometimes little children try to walk before the bones have become hard enough to support the weight of the body. This causes the legs to become crooked. In some countries young children work in factories and at various trades. This is wrong, because it dwarfs their growth, and makes them puny and sickly.
3. Improper Positions.—The bones are so soft and flexible when we are young that they are very easily bent out of shape if we allow ourselves to take improper positions in sitting, lying, or standing. This is the way in which flat and hollow chests, uneven shoulders, curved spines, and many other deformities are caused.
IMPROPER POSITION.IMPROPER POSITION.
4.In sitting, standing, and walking, we should always take care to keep the shoulders well back and the chest well expanded, so that we may not grow misshapen and deformed. Many boys and girls have ugly curves in their backbones which have been caused by sitting at high desks with one elbow on the desk, thus raising the shoulder of that side so high that the spine becomes crooked. The illustrations on this and the following page showgood and bad positions and also the effects of bad positions.