=How to fight the Flies.=--House flies lay at one time about one hundred eggs in the dirt thrown out of horse stables, in garbage cans, or in any other unclean place. In a day or two the eggs hatch into little white worms which feed on the dirt. One or two weeks later the worms change to flies.

Flies may be kept out of houses by putting screens in the windows and doors or by darkening the rooms when they are not in use. The few which gain entrance may be caught in fly traps. All food in the store or the home should be kept covered. It is not safe to eat candy on which flies have wiped their feet or to drink the milk in which they have washed them.

[Ill.u.s.tration: FIG. 80.--Photograph of a half handful of manure which had been thrown out of a horse stable. Note more than one hundred houseflies in the sleeping stage.]

The surest way to get rid of flies in any community is for all the people to work together and keep the entire neighborhood clean. No dead gra.s.s, weeds, or rags should be allowed to lie in the backyards or alleys. The cleanings from stables should be hauled away every week or stored in tightly covered boxes. Garbage cans must have close-fitting lids, so that there will be no place in which the young may hatch and grow.

=Other Insects which carry Disease.=--In certain parts of Africa, the _sleeping sickness_ has made ruins of prosperous villages. Thousands of the natives are dying yearly from this disease. The germs are carried from one person to another by the bite of a fly.

Some fleas carry the germs of _plague_, which a few centuries ago swept across Asia and Europe destroying hundreds of lives daily. The plague is now common in India and was present in California in 1908 and 1910. The bedbug spreads several kinds of fevers in warm countries and may also be a carrier of leprosy and typhoid fever. These facts show that insects are dangerous and should be kept out of the home.

Any one troubled with these little pests in the house may learn how to get rid of them by writing to the Department of Agriculture, Washington, D.C.

CHAPTER XVIII

HOW THE BODY MOVES

=The Need of a Framework.=--The body needs a stiff framework made of bones for three purposes. One purpose is to give it shape, a second purpose is to help the body move, and a third one is to protect from injury some of the delicate organs, such as the heart and brain.

The bones are nowhere separate but are joined together with tough bands named _ligaments_. All the bones together form the _skeleton_.

All animals from fish to man have a skeleton. Many of the lower creatures, such as worms and flies, have no bony skeleton. Most of these move sluggishly or have a hardened outer covering, like beetles and wasps. The skeleton of animals such as the cat, rabbit, or cow, has about the same number of bones as man, and they are arranged in the same way.

=Of what a Bone is Made.=--Although the bones are so hard, they are not dead. They contain blood, have feeling, and are just as much alive as the softer parts of the body. It is the lime that makes them stiff. This can be eaten out by putting the bone in strong vinegar or other acid for a few days. A long bone will then become so limber that it can be tied into a knot.

The living part of a bone can be burned out by placing it on hot coals for a half hour. At the end of this time the bone will look just as before, but when it is touched, will crumble to pieces.

=Forms of Bones.=--The bones of the legs and arms are hollow. This form gives the greatest strength with the least weight. You can prove this by using two sheets of paper. Roll one sheet and fold the other one. Hang weights on both ends of each and use the finger for a support in the middle.

The cavity of these bones is filled with a soft white substance called _marrow_. This is largely fat. Each bone is surrounded by a tough membrane to which the muscles are attached.

=Arrangement of the Bones.=--The bones of the head form the _skull_.

The other bones of the body not belonging to the _limbs_ make up the _trunk_. There are over two hundred bones in the entire body. Eight of these form a case for the brain. Fourteen give shape to the face. A chain of twenty-six bones named _vertebrae_ forms the backbone.

[Ill.u.s.tration: FIG. 81.--Photograph of the bones of the skeleton.]

Twelve pairs of _ribs_ encircle the chest. They are fastened behind to the backbone. The front parts of the ribs are made of pieces of gristle. The seven upper pairs are joined to the breastbone. The five lower pairs are named _false ribs_.

The _collar bone_ is in front of the shoulder and behind it is the flat _shoulder blade_. There is one bone in the upper part of each arm and leg and two bones in the lower part of each limb. Twenty-eight small bones are found in the hand, while twenty-seven are present in the foot.

=How the Bones may be Injured.=--In the young some of the entire bones and parts of many others are soft like gristle. For this reason, the bones of the young seldom get broken, but they are easily bent and pressed out of their natural shape. On this account you should hold the body erect in sitting and walking. Bending over the table or desk day after day is not only likely to cause round shoulders, but is sure to squeeze up the lungs and other organs so they cannot do their best work.

Sitting at a table or desk, so that one shoulder is higher than the other or carrying books at the side, so that they rest on the hip may cause a curve sidewise in the backbone. Tight clothing about the waist presses the ribs out of shape and hurts the other organs within the body.

[Ill.u.s.tration: FIG. 82.--How the bones are held together. A piece has been cut out of the tough ligament to show the cup of the hip bone into which the head of the thigh bone fits.]

=Caring for Broken Bones.=--When a bone of the arm or leg is broken, the muscles tend to make the ends shove over each other. The broken ends are sometimes sharp, and if the limb is bent, these may tear through the flesh. This may be prevented by binding a board firmly on opposite sides of the limb across the broken part. This will hold the bones in place until the surgeon comes and will also allow the patient to be moved.

The surgeon will set the broken bones by bringing the ends together and holding them in place by sheets of wood or metal firmly held by a bandage. In a few days the membrane around the bone begins to grow new bone to join the broken parts.

=How the Bones are joined together.=--The two general cla.s.ses of joints are the _movable_ and _immovable_. Except the lower jaw, the bones of the skull are so tightly joined together that there is no motion between them. The bones of the wrist and back have but little movement. The freest motion is at the shoulder joint, where the round head of one bone fits into the shallow cup of another. This is called a _ball and socket joint_. Such a joint is found also at the hip. At the elbow and knee the bones move back and forth like a hinge and these are named _hinge joints_.

=Working Parts of a Joint.=--The ends of the bones are covered with a thin layer of gristle. The bones are held in place by several strong bands called _ligaments_ (Fig. 82). These entirely surround the joint.

On their inner sides is a delicate membrane which gives out a slippery fluid to make the joint work easily.

The ligaments are sometimes strained, stretched, or torn by a fall.

The joint then swells because the watery part of the blood collects there. A sprained limb should be elevated to prevent swelling. Bathing it in very hot water is helpful.

=The Muscles.=--The muscles form the lean meat in any animal. They make up about one half the weight of the body. Each muscle is a bundle of thousands of little threads held together by other finer threads, while the whole is surrounded by a thin sheet. Little bundles formed of several of these threads called fibers may be seen in a piece of cooked beef picked to pieces. There are over five hundred muscles in the body.

[Ill.u.s.tration: FIG. 83.--Fifty of the muscles just under the skin.

Note the white cords, the tendons in the regions of the hands and feet.]

[Ill.u.s.tration: FIG. 84.--The biceps muscle contracted above and relaxed or loosened below.]

Some of the muscles are more than a foot long and have the shape of a ribbon. Some are circular like those around the mouth, eyes, and stomach, while others are large in the middle and taper toward the ends.

=How the Muscles are fastened to the Bones.=--The two ends of a muscle are attached to different bones. In many cases the muscle is not joined directly to the bone, but is connected to a tough white cord called a _tendon_. The tendon is then fixed to the bone.

Several of the muscles in the forearm run into tendons in the wrist because if the muscle part were to extend along the wrist, this part of the arm would be large and clumsy instead of graceful and slender.

Some of these tendons may be seen to move by bending the wrist and then working the fingers.

=How the Muscles do their Work.=--A tiny nerve thread runs from the spinal cord or brain to every muscle thread. Messages sent through the nerve threads to the muscles make them act. A muscle can act in only two ways (Fig. 84). It can become shorter or longer. When it gets shorter, we say it _contracts_. When it stretches out, it is said to _relax_.

A muscle cannot contract more than one fourth of its length. To pull the forearm up, the brain sends a message to the muscle fixed by one end at the shoulder and by the other end to a bone at the elbow. The muscle at once becomes shorter and thicker, as may be felt by placing the fingers on it. Although it shortens only two inches it is fastened to the bone so near the elbow that it draws the hand up two feet.

PRACTICAL QUESTIONS

1. Of what use are the bones?

2. What animals have bony skeletons?

3. What can you say of the form of bones?

4. How many bones in the body?

5. Name six bones.

6. What part of the arm has two bones side by side?

7. How many ribs have you?

8. Explain how a broken bone should be cared for.