Applied Physiology Part 12

3. If the eyes ache, the light should be softened or the position of the book or work changed, or else the eyes should be rested.

4. Sound in the air goes into the ear and strikes against a drum.

Bones then carry the sound to the ear nerves.

5. Air snuffed up the nose gives the sense of smell. Smell tells us if the air or food is fit for use.

6. Taste tells us whether food is fit for use. Men can learn to like the taste of wrong things like tobacco or alcohol.

[Ill.u.s.tration: =The Human Skeleton, showing position of bones.=]

CHAPTER XIV

BONES AND JOINTS

=180.= Bones make the body stiff and strong, and give it shape. Long bones reach through the arms and legs, and little bones reach down the fingers and toes. Rounded plates of bone form the head, and a pile of bony rings makes up the backbone. Each bone is built to fit exactly into its own place and to do its own work. In all there are over two hundred bones in the body. They form one seventh of its weight.

=181. Form of bones.=--A bone is not like a solid piece of timber, but is hollow like the frame of a bicycle. This makes it strong and light.

At its ends a bone is like a hard sponge covered with a firm sh.e.l.l.

This makes it too strong to be easily crushed, and keeps it light.

A bone grows like any other part of the body. It is made of living cells like woven threads. Lime is mixed among the cells, and makes them stiff like starch among the threads of a linen collar. Blood tubes go through every part of the bone so as to feed the cells. The living cells form one third of the bone, while the lime forms two thirds.

=182. Broken bones.=--Bones are very hard, and yet they can bend a little without breaking. Most of them are curved a little, and so they will spring instead of breaking when they are pressed hard. But sometimes they break. Then a person must wear a splint and bandage to keep the bones in place until they grow together again. The living cells will mend a bone in about a month.

An old person's bones are more tender than a child's, and will not spring much without breaking. An old man is afraid of falling and breaking his bones, while a child falls a dozen times a day without danger.

The bones of some children bend too easily. When they stand, the bones of their legs bend a little. After a while they grow in the crooked shape, and the child is bow-legged.

=183. Joints.=--Some bones are hinged upon each other. A bone hinge is a _joint_. The rings of the backbone are held together by very tough pads of flesh. Each pad lets the backbone bend only a little, but altogether they let us bend our backs in any direction. These pads are like rubber springs in a wagon, and keep our bodies from being jarred too much.

The finger and toe joints, the wrists and ankles, the elbows and the knees, bend back and forth like a hinge. Tough bands of flesh bind the bones together. The ends of the bones are rounded and smooth. They fit together and make perfect hinges. The joints are oiled by a fluid like the white of an egg. In old people this fluid sometimes dries up. Then the joints become stiff, and creak like a squeaking hinge.

[Ill.u.s.tration: =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 a _sprain_. 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.

WHAT WE HAVE LEARNED

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.

[Ill.u.s.tration: =The muscular system.=]

CHAPTER XV

MUSCLES

=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 pa.s.ses over a joint, and becomes fast to another bone. You can easily feel the tendons in the wrist and behind the knee.

[Ill.u.s.tration: =Muscle cells, cut across (200).=

_a_ muscle cell.

_b_ connective tissue binding the cells 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.

[Ill.u.s.tration: =A thin slice of a voluntary muscle, cut lengthwise (100).=

_a_ muscle cell.

_b_ capillaries surrounding the cells.

_c_ connective tissue binding the cells 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.