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 vertebræ forms the backbone.
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.
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 classes 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.
Fig. 83 —Fifty of the muscles just under the skin. Note the white cords, the tendons in the regions of the hands and feet.
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.
9. Point out and name two kinds of joints.
10. What are ligaments?
11. Of what is a muscle made?
12. How many muscles in the body?
13. How many tendons can you feel in your wrist?