The elbowHinge joint of the elbow.1 humerus 2 ulna
Hinge joint of the elbow.1 humerus 2 ulna
The shoulders and hips can be moved in every direction. The upper ends of the arm and leg bones are round like half a ball. They fit into cups on the shoulder and hip bones. They are very smooth, and are oiled like the hinge joints. The joints are made to work very smoothly and easily.
184. Bones out of joint.—When the ends of bones are torn away from each other, the bone is out of joint. Then the bone cannot be moved without great pain. It should be put back in place at once and kept there by splints and bandages. A person is less liable to have his joints out of place than he is to have his bones broken.
185. Sprains.—Sometimes a joint is turned too much. This stretches the flesh around the joint, and makes it very tender and painful. This is asprain. When you sprain a joint, you should put it in hot water for an hour or two. Then keep it still for a few days.
186. Why bones and joints grow wrong.—While bones and joints are growing they can be made to take any shape we please. They cannot be bent all at once, but if we hold them in one way much of the time, they will keep that shape. Some boys and girls sit with their backs bent forward and lean against the desk as if they were too lazy to sit up. When they grow up, they will be bent and round-shouldered. You should sit and stand straight. Then you will grow tall and straight and strong. A soldier has square shoulders and walks erect because he is drilled until his bones and joints grow in the proper shape. As you stand straight with your feet together, your two big toes, your two ankles, and your two knees should touch each other.
If you wear tight shoes and press the toes out of shape, they will soon grow so. Nearly every one's feet are out of shape from wearing short, pointed shoes. Your toes should be straight and not cramped by the shoe. If you wear narrow shoes,you may harm your feet. It is better to have one's feet useful, even if they are large, than to make them small and useless.
1. Bones make the body stiff, and give it form.2. Some bones are long, some round, and some flat. All are hard and springy.3. Some bones are hinged together. The hinge is a joint.4. The ends of bones in joints are rounded and smooth, and are oiled with a liquid like the white of an egg.5. Some bones are bound together by springy pads, as in the backbone.6. Bones can be broken. They will grow together again themselves.7. Joints can be put out of place; then we must put them back.8. If joints or bones are kept in wrong positions they will grow into bad shapes. Tight shoes deform the feet.
1. Bones make the body stiff, and give it form.
2. Some bones are long, some round, and some flat. All are hard and springy.
3. Some bones are hinged together. The hinge is a joint.
4. The ends of bones in joints are rounded and smooth, and are oiled with a liquid like the white of an egg.
5. Some bones are bound together by springy pads, as in the backbone.
6. Bones can be broken. They will grow together again themselves.
7. Joints can be put out of place; then we must put them back.
8. If joints or bones are kept in wrong positions they will grow into bad shapes. Tight shoes deform the feet.
The body's musclesThe muscular system.
The muscular system.
187. Shape of muscles.—Bones are covered with muscles. Muscles give shape to the body, and move it about. One half of the body consists of muscles. These are arranged in bundles, and each causes a bone to make one motion. There are over four hundred separate bundles of muscle in the body.
One end of a muscle is large and round and is fast to a bone. The other end tapers to a strong string or tendon. The tendon passes over a joint, and becomes fast to another bone. You can easily feel the tendons in the wrist and behind the knee.
CellsMuscle cells, cut across(×200).amuscle cell.bconnective tissue binding thecells together.
Muscle cells, cut across(×200).amuscle cell.bconnective tissue binding thecells together.
A muscle is made of tiny strings. You can pick them apart until they are too fine to be seen with the eye. Each string is a living muscle cell. It is the largest kind of cell in the body. You can see the fine strings in cooked meat.
Slice of muscleA thin slice of a voluntary muscle,cut lengthwise (×100).amuscle cell.bcapillaries surrounding the cells.cconnective tissue binding thecells together.
A thin slice of a voluntary muscle,cut lengthwise (×100).amuscle cell.bcapillaries surrounding the cells.cconnective tissue binding thecells together.
188. How muscles act.—A nerve runs from the brain, and touches every cell of the muscle. When we wish to move, the brain sends an order down the nerve. Then each muscle cell makes itself thicker and shorter. This pulls its ends together, and bends the joint. We can make muscle cells move when we wish to, but we cannot make any other kind of cell move. We make all our movements by means of our muscles.
189. Where you can see muscles.—In a butcher's shop you can see lean meat. This is the animal's muscle. White and tough flesh divides the tender red meat into bundles. Each red bundle is a muscle. You will see how the muscle tapers to a string or tendon. The butcher often hangs up the meat by the tendons. You can see the muscles and tendons in a chicken's leg or wing when it is being dressed for dinner.
Roll up your sleeve to see your own muscles. Shut your hand tight. You will see little rolls under your skin, just below the elbow. Each roll is a muscle. You can feel them get hard whenyou shut your hand. You can feel their tendons as they cross the wrist.
Open your hand wide. You can see and feel the tendons of the fingers upon the back of the hand. These tendons come from muscles on the back of the arm. You can feel the bundles of these muscles when they open the fingers. There are no muscles in the fingers, but all are in the hand or arm. You cannot open your hand so strongly as you can close it.
190. Strength of muscle.—By using a muscle you can make it grow larger and stronger. If you do not use your muscles they will be small and weak. Children ought to use their muscles in some way, but if they use them too much, they will be tired out. Then they will grow weaker instead of stronger. Lifting heavy weights, or running long distances, tires out the muscles, and makes them weaker. Small boys sometimes try to lift as much as the big boys. This may do their muscles great harm.
191. Round shoulders.—The muscles hold up the back and head, and keep us straight when we sit or stand. A lazy boy will not use his muscles to hold himself up, but will lean against something. He will let his shoulders fall, and will sit down in a heap. Sometimes he is made to wear shoulderbraces to keep his shoulders back. This gives the muscles nothing to do, and so they grow weaker than ever. The best thing to do for round shoulders is to make the boy sit and stand straight, like a soldier. Then he will use his muscles until they are strong enough to hold his shoulders back.
192. How exercise makes the body healthy.—When you use your muscles, you become warmer. Your face will be red, for the heart sends more blood to the working muscle cells. You will be short of breath, for the cells need more air. You will eat more, for your food is used up. Your muscles are like an engine. They get their power from burning food in their own cells. When they work they need to use more food and air. So working a muscle makes us eat more and breathe deeper. The blood flows faster, and we feel better all over. The muscle itself grows much larger and stronger.
If we sit still all day, the fires in our bodies burn low and get clogged with ashes. We feel dull and sleepy. If we run about for a few minutes, we shall breathe deeply. The fires will burn brighter. Our brains will be clearer, and we shall feel like work again. Boys and girls need to use their muscles when they go to school. Games and play will make you get your lessons sooner.
193. How to use the muscles.—You should use your muscles to make yourself healthy, and not for the sake of growing strong. Some very strong men are not well, and some men with small muscles are very healthy. Some boys have strong muscles because their fathers had strong muscles before them. Strength of muscle does not make a man.
You ought to have healthy muscles. Then your whole bodies will be healthy, and you can do a great deal of work. You ought to learn how to use your muscles rather than how to make them strong. An awkward and bashful boy may be very strong, but he cannot use his muscles. A boy is graceful because he can use them.
The best way to use your muscles is in doing something useful. You can help your mother in the house and your father at the barn. You can run errands. You can learn to use carpenter's tools or to plant a garden. Then you will get exercise and not know it. You will also be learning something useful.
Play is also needed. Work gets tiresome, and you will not want to use your muscles. Play is bad when it takes you from your work or when you hurt yourself trying to beat somebody.
194. Alcohol and the muscles.—Men use alcoholto make themselves strong. It dulls their weak feelings, and then they think themselves strong. They are really weaker. The alcohol hinders digestion and keeps food from the cells. Then the fires in the body burn low, and there is little strength.
Alcohol sometimes causes muscle cells to change to fat. This weakens the muscles.
Men sometimes have to do hard work in cold countries; and at other times they must make long marches across hot deserts. Neither the Eskimos in the cold north, nor the Arabs in the hot desert, use strong drink. Alcohol does not help a man in either place. It really weakens the body. The government used to give out liquor to its soldiers; but soldiers can do more work and have better health without liquor and it is no longer given out.
A few years ago men were ashamed to refuse to drink. Even when a new church building was raised, rum was bought by the church and given to the workmen. Farmers used to give their men a jug of rum when they went to work. Farm hands would not work without it.
Now all this has changed. Men do not want drinkers to work for them. A railroad company will discharge a man at once if he is known to drink at all. A man can now refuse to drink anywhere and men will not think any less of him.
195. Tobaccopoisons the muscle cells and makes them weak. At first it makes boys too sick to move. It always poisons the cells even if they do not feel sick.
196. A long life.—A man's body is built to last eighty years, but only a few live so long. If you are careful in your eating and drinking, if you breathe pure air, and if you use your muscles, your body will be healthy and will last the eighty years and more. All through your life you will be strong and able to do good work.
1. Muscles cover the bones and move the body.2. Muscle is lean meat. It is made of bundles of cells like strings. Nerves from the brain touch each cell.3. Each muscle is fast to a bone. It becomes a small string or tendon at the other end. The tendon crosses a joint and is fast to another bone.4. When we wish to move, the brain sends an order to the muscle cells to make themselves thicker and shorter and so bend the joint.5. You can feel the muscles and tendons in the arm and wrist.6. Muscle work makes us breathe deeper, and eat more food. It makes the blood flow faster. So it makes our whole bodies more healthy.7. Every one ought to use his muscles some part of the day.8. Alcohol and tobacco lessen the strength of the muscles.
1. Muscles cover the bones and move the body.
2. Muscle is lean meat. It is made of bundles of cells like strings. Nerves from the brain touch each cell.
3. Each muscle is fast to a bone. It becomes a small string or tendon at the other end. The tendon crosses a joint and is fast to another bone.
4. When we wish to move, the brain sends an order to the muscle cells to make themselves thicker and shorter and so bend the joint.
5. You can feel the muscles and tendons in the arm and wrist.
6. Muscle work makes us breathe deeper, and eat more food. It makes the blood flow faster. So it makes our whole bodies more healthy.
7. Every one ought to use his muscles some part of the day.
8. Alcohol and tobacco lessen the strength of the muscles.
197. Catching diseases.—Our body may get out of order like a machine. Some parts of it may be cut, or broken, or worn out, or hurt in other ways. Then we are sick until it is made whole again. Sickness always means that a part of the body is out of order.
Some kinds of sickness are like a fire. A small bit of something from a sick person may start a sickness in us, just as a spark may set a house on fire. Then we may give the sickness to others, just as a fire may spread to other houses. If a person has measles, we may catch the measles if we go near him; but if a person has a toothache, we cannot catch the toothache from him. So we may catch some kinds of diseases, but we cannot catch other kinds.
198. Bacteria and germs.—Every kind of catching sickness is caused by tiny living things growing in our flesh and blood. Some of them are tiny animals. Most of them are plants, and are calledbacteriaormicrobes. A common name for all of them isgerms.
The word germ means nearly the same as the word seed. Bacteria are so small that we cannot see them unless we look at them through a strong microscope. Then they look like little dots and lines (p.54). A million of them could lie on a pin point; but if they have a chance, they may grow in numbers, so that in two days they would fill a pint measure.
Very many kinds of bacteria and other germs are found nearly everywhere. They are in the soil and in water, and some float in the air as dust. When they fall on dead things, they causedecayorrotting. When we can fruit, we kill the germs by boiling the fruit and the cans. Then we close the cans tightly so that no new germs can get into them. The fruit will then keep fresh for years.
Decay is nearly always a good thing, for by it dead bodies and waste substances are destroyed and given back to the ground, where plants feed upon them. Many plants would not grow if they could not feed upon decaying things. So most bacteria and other germs are useful to us. But some kinds of germs will grow only in our bodies, and these kinds are the cause of most of our sickness.
199. Germs of sickness.—We catch a sickness by taking a few of the germs of the sickness into our flesh. There they grow quickly, like weed seeds inthe ground, and form crops of new germs within a few hours. After a few days the germs become millions in number, and crowd the cells of our flesh, just as weeds may crowd a potato plant (p.54).
Disease germs in the body also form poisons, just as some weeds in a field form poisons. The poisons make us sick, just as if we had swallowed the leaves of a poisonous weed.
200. Fever.—If a sickness is caused by disease germs, the body is nearly always too warm. Then we say that the sick person has afever. Almost the only cause for a fever is disease germs growing in the body. We can make a person have any kind of fever by planting a few of the germs of the fever in the right part of his body.
We are made sick by the germs of fevers more often than by all other causes put together. Here is a list of common diseases caused by fever germs:—colds and sore throats, most stomach aches, blood poisoning in wounds, boils and pimples, tuberculosis, whooping cough, measles, chicken pox, diphtheria, scarlet fever, typhoid fever, smallpox, and malaria.
Which of these kinds of sickness have you had? What sickness have you had besides these?
201. Sickness and Dirt.—Disease germs leave the body of a sick person in three ways: first, through the skin, second, through the kidneys and intestines, andthird, through the nose and throat. In these same ways our body gives off its waste matters. If we did not take anything from another person's body into our own body we should not catch fevers.
Whatever a feverish person soils may contain disease germs. When a person has only a slight fever he often keeps at work, and then he may scatter disease germs wherever he goes. So disease germs are likely to be found wherever there is dirt or filth. Cleanliness means good health as well as good looks.
202. Disease germs in the skin.—Disease germs may often be found in sores and pimples on the skin, but they will not leave anybody's flesh and blood through sound and healthy skin. If our skin is smooth and fair, there will be few disease germs on it unless we rub against something dirty. A dirty skin nearly always contains disease germs. Washing and bathing our body will take disease germs from our skin and help us to keep well.
203. Disease germs in slops.—A great many disease germs leave the body through the intestine and kidneys, and may be found in the slops and waste water of our houses. Slops are dangerous to health, for they may run into a well, or spring, or river, and so carry disease germs into our drinking water (p.27). Also, house flies may light on the pailsor puddles and carry the germs to our food. In these ways we catch typhoid fever, stomach aches, and other diseases of the intestines. All slops and waste matters from the body should be put where they cannot reach our drinking water, and where flies cannot crawl over them (p.80).
204. Disease germs from the nose and throat.—If a person is sick with a fever, many of the germs are likely to be found in his nose and throat. Thousands of them are driven out with every drop of saliva and phlegm when he blows his nose, or spits, coughs, or sneezes, or talks. If he puts anything into his mouth, it will be covered with germs. More diseases are spread from the nose and mouth than in any other way, for we are always doing something to spread bits of saliva and phlegm.
205. Spitting.—Colds and consumption and other forms of sickness are often spread by sick persons spitting on the floor or pavement. The germs become dried and are blown away as dust. For this reason dust from the streets of cities and in crowded halls is often the cause of sickness. In many places spitting on a floor or pavement is strictly forbidden by law.
206. Putting things in the mouth.—Many persons have the habit of sucking their fingers, or of touching a pencil to the tongue when they write or think,or of wetting their fingers with their lips when they turn the leaves of a book. In all these ways we may give a disease to others or may take a disease from some one else.
207. Public drinking cup.—When you touch your lips to a cup, you leave some saliva and cells from your mouth on the cup. If a cup is used by a number of persons, some one is almost sure to leave germs of sickness on it, and others are likely to take them into their own mouths when they drink. So a public drinking cup is a dangerous thing. Each school child should have his own cup. Public drinking fountains should be so made that we may drink by putting our lips to a stream of running water.
Drinking placesA safe drinking fountain.An unsafe drinking place.A stream of water gushes up from themiddle of the cup.Photograph taken in the basement ofa schoolhouse.
208. Sweeping.—Dusty air in a room is dangerous to health, for disease germs are likely to be found in it. We can get rid of dust by keeping our floors swept clean. After sweeping we should wipe the dust from the tables and furniture. A feather duster or dry cloth will only stir up the dust and make it float in the air again. We should use either a damp cloth, or a dry duster made of tufts of wool, so that the dust will stick to the duster.
A flyHouse fly, magnified.The hairs on its body and legs catchdirt and disease germs.
House fly, magnified.The hairs on its body and legs catchdirt and disease germs.
209. Foul air.—If we live in a closed room, the air soon becomes foul and dusty, and is likely to have disease germs in it. Foul air is one of the greatest of the causes of sickness. We should change the air of a room often so as to keep it fresh and free from dust and disease germs (pp.65–67).
210. House flies.—House flies come from garbage heaps and filth of all sorts. So they carry diseasegerms on their bodies. They light on our food and on our faces, and so they often make us sick. They are often the cause of typhoid fever, stomach aches, and stomach sickness in babies.
Life cycleLife history of house flies.
Life history of house flies.
Flies are hatched in manure piles and garbage heaps. At first they look like white worms, and are calledmaggots. Every maggot is a young fly. We can get rid of flies by cleaning up every garbage heap and manure pile.
The youngYoung mosquitoes hanginghead downward in water.
Young mosquitoes hanginghead downward in water.
211. Mosquitoes.—Mosquitoes carry malaria and yellow fever from sick persons to the well. If there were no mosquitoes, there would be no malaria or yellow fever.
Mosquitoes are hatched in water, and the young are calledwigglers. We may often see them in rain barrels. We may get rid of mosquitoes by emptying all rain barrels and pails and cans of dirty water, at least once a week, and by drying up swamps and marshes.
1. We catch a fever by taking disease germs into the body.2. Disease germs cannot be seen without a strong microscope.3. The germs may be found in dust and dirt.4. Slops from our houses are often full of the germs.5. You may take germs into your body by putting pencils and other things into your mouth, and by drinking from a public drinking cup.6. Spitting on the floor or pavement may scatter disease germs.7. House flies and mosquitoes often spread diseases.
1. We catch a fever by taking disease germs into the body.
2. Disease germs cannot be seen without a strong microscope.
3. The germs may be found in dust and dirt.
4. Slops from our houses are often full of the germs.
5. You may take germs into your body by putting pencils and other things into your mouth, and by drinking from a public drinking cup.
6. Spitting on the floor or pavement may scatter disease germs.
7. House flies and mosquitoes often spread diseases.
212. How our body kills disease germs.—We take disease germs into the body in three ways: first, through the mouth, second, through the nose, and third, through the skin. So we should watch the purity of our food, drink, and air, and should be careful about putting things into the mouth, and about the cleanliness of the skin. We often take a few disease germs into the body without catching a disease. This is because the white cells of our blood fight the germs and kill them (p.53). If the body is hurt or weakened, the white blood cells may also be weakened so that they cannot kill the germs. We should take good care of the body so that every part of it may do its work well. We need not be able to run fast, or to lift heavy weights, but the best sign that every part of the body is in good order is to feel bright and wide-awake. Then our white blood cells will also be in good order and able to fight disease germs.
213. Catching cold.—When we catch a disease, we often say that we have caught cold. We used tothink that cold air and dampness were almost the only causes of taking cold, and this is the reason why we called many kinds of sickness by the name of colds. Now we know that we catch cold by taking disease germs into the body. The germs will not be able to grow unless the body is weakened in some way, as by cold and dampness. Yet if we are wet and cold, we shall not catch cold unless we take disease germs into the body. We do not get the germs from the outdoor air, for very few germs are there. We get them from the foul air of our houses when we come in to warm and dry ourselves. If the air of our houses were always as clean and pure as the outdoor air, we should hardly ever have colds.
We can safely let the cold air blow on us if we are out of doors, but if we sit in a house, a small draft sometimes seems to make us take cold. This is because there are likely to be many disease germs in the house and few out of doors.
Other things besides cold air and dampness may weaken the body, and so help us to take cold. If germs of colds are in a warm room, we may sit there and take cold even if we are not wet or chilled at all. The body may be weakened by poor food, wrong eating, or overwork, so that disease germs will easily grow in it. We take as many colds from these causes as from cold air and dampness.
214. Kinds of colds.—A person takes most of the germs of colds through his nose and mouth. If they grow only in his nose, we say that he has a cold in his head. If they grow in his throat, he has a sore throat, or tonsillitis. If they reach as far as the upper part of his windpipe, he is hoarse, or has a cough, or the croup. If the germs are planted in his lungs, he may have bronchitis or pneumonia. All these kinds of sickness often spread from one person to another. If one person in a family has a cold, others in the family often catch cold from him.
215. Diseases like colds.—Diphtheria, tuberculosis, whooping cough, and measles all begin like a common cold and often look like a cold during the whole sickness. Colds do not turn into any of these diseases, for each of them comes from its own germ, just as corn comes only from seed corn.
216. Curing a cold.—If you have a cold, you ought to stay at home and rest, or lie in bed. Then your white blood cells can gain strength to fight the disease germs. You ought to have plenty of fresh air in your room. You ought not to eat much food for a few days, so that your stomach and intestine and liver can use all their strength in throwing off the poisons of the germs. But you ought to drink plenty of water, so as to help wash away the poisons from your body.
217. Keeping colds from spreading.—You should keep away from other persons while you have a cold, or other catching disease, so as to keep from spreading the sickness. You ought not to go visiting, or go to school, or to church, or to other meeting places. When you cough or sneeze, you should hold a handkerchief to your mouth, so as to keep from blowing disease germs from your throat and nose. You ought to sleep in a bed by yourself, so that no one may take the disease germs from your bedclothes. No one else should use your towel, or handkerchief, or knife, or fork, or spoon, or dish, until they have been washed in hot water, so as to kill the disease germs on them.
218. Keeping from catching cold.—You can keep yourself from catching cold by keeping your body strong and in good order. You should keep your clothes dry, eat good food, breathe pure air, get good rest and sleep, and keep your body, your clothes, and your house clean. You should also keep disease germs out of your body. You should not form a habit of putting your fingers or a pencil to your mouth (p.127). You should keep your nose, your throat, and your mouth clean.
219. Cleanliness of the nose.—The inside of the nose is wet with a slippery liquid. If you have a cold, the liquid is thick and stops your nose, and is calledphlegm. The liquid catches and holds dust and disease germs, and keeps them from going into the windpipe. It also kills many of the disease germs.
You should always carry a handkerchief and use it so as to blow the germs out of your nose. You should have a clean handkerchief every day.
View of nose and throatPhotograph of model of thenose and throat.A.tonsil;B.adenoids;C.opening of Eustachian tube.
Photograph of model of thenose and throat.A.tonsil;B.adenoids;C.opening of Eustachian tube.
220. Adenoids and large tonsils.—Sometimes children have large tonsils growing in the back of the throat, or soft bunches of flesh calledadenoidsback of the nose. These children cannot breathe well through the nose, but must breathe through the mouth. Then they take dust and disease germs deep into the body, and so take colds and other sickness easily. If a child has adenoids or large tonsils, an operation should be done to take them out.
221. Cleanliness of the mouth.—We often breathe dust and disease germs into the mouth or snuff them into the throat from the nose. Then they are caught between the teeth and in the folds of the cheeks and throat. There they may grow, and finally go deeper into the body and make us sick. A dirty mouth is very often the cause of colds and other sickness.
We should keep our mouths clean by brushing our teeth with a toothbrush two or three times a day. We should also rub the toothbrush over the tongue and around the back part of the throat so as to clean the germs from every part of the mouth. Each child should have a toothbrush of his own, and should use it every day.
222. Contagious diseases.—Diphtheria, whooping cough, measles, scarlet fever, and smallpox are all dangerous kinds of sickness, and spread with great ease. The germs may float in the air, and we may take them into our bodies if we go into a room where any one has the sickness. So we call these diseasescontagious. If a person has one of these diseases, he should be made to stay in a house or room by himself until he is well. Keeping the sick away from well persons is calledquarantine. When the sickness is cured, the sick room and everything in it should be cleaned and washed so as to kill the germs.
223.Board of health.—There is a board of healthin every city and town. The men on the board show persons how to keep diseases from spreading, and make them obey the rules of health. Everybody in a town should help the board of health in every possible way.
1. The white blood cells of our body kill disease germs.2. We catch cold by taking disease germs into our body.3. The germs of colds are not often found in the air out of doors. They are often found in the foul air of houses.4. If a person has a cold, he should keep away from other persons, so as to keep from spreading the sickness.5. Cleansing the nose helps us to keep from catching cold.6. Cleansing the teeth and the inside of the mouth removes many disease germs.7. Adenoids and large tonsils should be taken from the throat by an operation.8. If a person has a dangerous contagious disease, he should be quarantined.9. Boards of health have charge of the prevention of contagious diseases.
1. The white blood cells of our body kill disease germs.
2. We catch cold by taking disease germs into our body.
3. The germs of colds are not often found in the air out of doors. They are often found in the foul air of houses.
4. If a person has a cold, he should keep away from other persons, so as to keep from spreading the sickness.
5. Cleansing the nose helps us to keep from catching cold.
6. Cleansing the teeth and the inside of the mouth removes many disease germs.
7. Adenoids and large tonsils should be taken from the throat by an operation.
8. If a person has a dangerous contagious disease, he should be quarantined.
9. Boards of health have charge of the prevention of contagious diseases.