The skeleton or bony system, as we have seen, furnishes a strong framework or support to the body, constructed in such a way that its parts can move and thus help in the motion of the body as a whole. The skeleton also serves to protect certain parts of the body from injury, as in the case of the skull and the ribs.
What the Bones are Made Of. Bone consists largely of lifeless matter, but it has all been built up out of living tissue. In a very young child, the framework of the body is made of a soft material called cartilage. Gradually the cartilage changes to bone, as lime is added to it from the tissues. This makes the bones hard, just as lime gives hardness to clam shells and oyster shells.
All bone consists of hard lime and some softer sub-stances. We can prove this in several ways. If we heat a piece of bone for a long time in the fire, it will become so brittle that at a touch it falls into powder. This is because all the soft connecting material has been burned out, leaving nothing but the lime. A cook extracts much of the softer part of bones when she boils them for several hours, the soft part (called gelatin) being used in making soups. The lime, on the other hand, may be taken out of a bone with acid, leaving the bone so flexible that it can be tied into a knot.
Some of the larger bones, which are hollow, have a soft fatty material called marrow in the center.
The Bones in Childhood and Old Age.All through childhood, the bones are very flexible and contain more living matter than in later life. When a baby first stands on his feet, it is easy to see that the bones in his legs are not stiff and strong. If a child is al-lowed to walk too early, before the cartilage has hardened into bone, his legs become bent out of shape. There is danger that the bones will keep this bent shape and the child may become permanently bow-legged.
The bones of an old person, on the other hand, have less of the soft living matter. They are dry and brittle and break easily. A very slight fall, which would hardly be noticed by a child, will sometimes snap the bones of a person of advanced years.
Parts of the Skeleton.The human skeleton includes about two hundred separate bones, twenty-eight of which are in the head, fifty-eight in the trunk, and one hundred and twenty in the arms and legs.
The backbone is a kind of column which supports the upper part of the body. Above it is the skull, which incloses and protects the brain. The ribs and the breastbone surround the organs in the upper part of the trunk, and a group of hip bones, called the pelvis, help to support the organs in the lower part of the body. The arms and the legs are given strength by a series of long slender bones which run like rods through them.
Shapes of the Bones.The bones are of various shapes, and all are admirably suited for the work they have to do. The ribs are curved so as comfortably to inclose the organs of the upper part of the trunk. The bones of the skull are flattened and curved plates, which form a tight box to hold the brain. In the joints of the wrists and ankles, there are a number of small rounded bones, somewhat like the ball bearings in the wheels of a bicycle. The large bones in the arm and the leg have the shape of hollow cylinders, a form which makes them light yet gives great strength. (See Fig. 9.)
How the Bones are Held Together.The bones are held together at their joints by tough bands or cords of tissue called ligaments. The muscles and tendons (to be described in Chapter IV) also serve to bind together the whole bony system.
Some of the bones are fixed firmly to those that lie next them; this is the case in the skull. Others, like the bones in the arm, are connected. by movable joints. At these movable joints, the ligaments hold the bones in place, while allowing them a certain freedom of motion. Between the bones, in many cases, there are layers of cartilage, like the substance of which the whole skeleton is formed at first. In some of the movable joints, there is still another arrangement to make the bones move easily. These joints are covered by little bags filled with a liquid that keeps the ends of the bones wet, so that they move freely over each other, acting somewhat like the oil that we put into a hinge to keep it from creaking.
Kinds of Joints.There are several different forms of joints which make possible various kinds of movement. The joint in the elbow is of the simplest kind, in which a bone moves up and down in two opposite directions but not sideways, just as a knife blade does in opening or shutting. The knee joint is of the same kind, and there are a number of others that you can find for yourself. Such joints are called hinge joints.
The joint in the shoulder is a different kind, which allows much freer motion in all directions. This is called a ball-and-socket joint, because one of the bones has a rounded knob or ball which fits into a cup or socket in the other bone and moves about in it.
Finally, there is a third kind of joint, called a gliding joint, in which the bones move or glide over each other in different directions but moving only a little each way. Such joints are found in the wrists and the ankles.
The Spinal Column.The backbone or spinal column is very strong so that the body may be held firmly, and yet quite flexible so that it may bend easily forward or backward or from side to side. It is made up of twenty-four distinct and separate ring-shaped bones, called vertebrae (ver’ te bre) with nine or ten smaller bones at the lower end, which are solidly attached to each other. The vertebrae are piled one above another in a long column slightly curved from front to back. They are bound together by ligaments and by many small muscles. When the muscles on one side of the column contract, the ligaments on the other side stretch and the vertebrae tilt slightly on each other.
The vertebral rings have large prongs or teeth, to some of which the ribs are attached. Through the center of the rings runs the spinal cord, which is thus carefully protected from injury,a very important thing, since the cord is as necessary for the life of the body as the brain itself.
The backbone, when seen from the side, is not straight but is somewhat S-shaped. This form allows a freedom of movement which the body would not have if the backbone were simply a straight column.
The Ribs, Breastbone, and Pelvis.The principal organs of the upper part of the trunk are inclosed in the bony cage formed by the twelve pairs of hooplike ribs. The ribs are joined to the spinal column at the back, and all but the lowest two pairs are joined at the front to the breastbone or sternum. The whole set of ribs can be slightly raised or lowered by a special group of muscles. You can feel this motion of the ribs when you draw a deep breath. The hip bones, which together form the pelvis, make a bowl-shaped support for the organs in the lower part of the trunk. (See Fig. 6.) These organs are supported also by powerful muscles in the front wall of the abdominal cavity.
The Skull.Eight bones, firmly joined together, make a strong, rounded case or box, called the skull or cranium, in which rests the brain. The edges of these bones are irregular and fit each other tightly. At the base of the skull, there is a large opening through which the spinal cord enters from the backbone. In the front are openings through which pass various nerves, including those from the eyes. The jaw bones, which hold the teeth, and the bones of the face are attached to the cranium.
The Bones of Arms and Legs.The number of bones in each arm and each leg is the same (thirty), and the arrangement of twenty-nine of them is the same in each case. There is one bone in the wrist not present in the ankle, and the front of the knee has a protecting bone (the knee-cap) not found in the elbow.
The bones of the arm are attached at the shoulder by the large, broad shoulder blade (scapula) and the clavicle or collar bone, which is a delicate bone often broken by a fall. You can feel your collar bone extending from the breast-bone to the shoulder. In a similar way, the bones of the legs are joined to the lower part of the backbone by the broad and strong pelvis.
In both arm and leg, there is a single bone running down to the elbow or knee, called the humerus in the arm and the femur (fe mur) in the thigh. Below the elbow or knee there is a pair of long parallel bones running to the wrist or ankle, called the radius and ulna (ul na) in the arm and the tibia (tib’ i a) and fibula (fib’ u la) in the leg.
In the wrist there is a group of eight small bones, and in the ankle there are seven. In the palm of the hand and the instep of the foot there are five parallel bones, which connect with rows of two or three bones in each finger and toe.
The joints of the arms and legs are excellent mechanical devices. The humerus has, at the upper end, a rounded knob which fits into a socket in the shoulder blade and turns easily, while its lower end has sockets in which the two bones of the forearm move. Similarly, the femur is attached at the top to the pelvis, and its lower end is rounded and fits into the bones of the lower leg in such a way as to permit the knee to move easily.
The movements of the wrist and the ankle, in which a number of small bones play a part, are even more complicated than the movements of shoulder and elbow or hip and knee, and the fingers are capable of the most complex movements of all. The toes of young babies move almost as easily as their fingers, but the toes of grown people usually lose this power in greater or less degree.
Holding the Body Well.As the skeleton is a flexible framework, it may take various positions, which are good or bad in their effect on the body as a whole. Posture depends on the muscles which control the position of different parts of this framework. With the same equipment of bones, one body may be stoop-shouldered and slouching and another may be erect and well-knit. One of the first things to learn in managing the human machine is to hold the body well.
The backbone, as we have seen, is meant to be slightly curved, so as to give elasticity. In people who do not sit or stand straight, these curves become greatly exaggerated, leading to round shoulders and a drooping head.
Bad posture is not only ungraceful but unhealthy. If the back and shoulders are not held properly, the lungs and other internal organs do not get a sufficient supply of blood.
If a good posture is not maintained, the muscles develop unequally, and it becomes more and more difficult to hold oneself well. Among growing children, correct posture is therefore particularly important. Every boy and girl who wishes to have a strong and graceful body should keep the back straight, the shoulders square, and the head high.
The Correct Position for Standing and Sitting.When standing, the head, body, and legs should be poised one above the other so that a line dropped from the front of the ear falls within the forward half of the foot. The shoulder blades should be flat across the back and the feet should be directed straight forward (not outward). “This is the position of the long-distance walker, the mountain climber, the best all-round athletes; it is the position of command and authority and is found predominant in the great leaders of commerce and public life. On the other hand, collapsed positions are characteristic of both physical and mental weakness. They constitute a distinct aspect of weakness and illness, from the tuberculosis patient to the feeble-minded.”
In sitting, the body should be bent only at knees and hips, and the head, neck, and trunk should be kept in one straight line.
Stand in front of the mirror some day in your ordinary resting posture and see how you look. Notice whether your head is well up or bent forward, whether your shoulders are square or bowed, your chest rounded or flat, your knees sagging or straight, your feet set squarely on the ground and side by side, or tilted over on weak ankles and toeing out or in.
Some Causes of Faulty Posture.One bad deformity of the backbone is called lateral curvature. The backbone seen from front or back should be straight. Sitting at a school desk which is too low, or too near the seat, may lead not only to round shoulders, but to a bending of the back-bone to one side or the other. If seats in the schoolroom are adjustable, such harmful effects may be avoided. When there are no adjustable seats, a wooden support screwed under desk or chair makes it possible to raise one or the other to fit the pupil’s needs. If both seat and desk are too high, a board or some bricks may be placed on the floor for a footrest.
Another thing that strains the body out of shape is the frequent carrying of a heavy weight on one arm. School-books and heavy bundles should be divided and carried some on one arm and some on the other.
Tight clothing may do serious damage by cramping the movement of the ribs and getting them out of shape. It may twist the organs out of place and crowd them together, so that they cannot develop and do their work properly. It may injure the breathing muscles and seriously affect the stomach, the intestines, and the liver.
Hygiene of the Feet.The weight of four-footed animals is divided almost evenly and produces very little strain on the feet. Man’s upright position, which places the whole weight of the body on two feet, causes a much greater strain, especially at ankle and instep.
The healthy normal foot has a well marked arch under the instep, and this arch is very important in giving springiness or elasticity to the step. When the arch is broken down, as in the deformity known as flat-foot, the nerves and blood vessels underneath are injured. Pains far up the back may be caused by the resulting strains. Exercise of the muscles which move the toes, if begun in time, will often correct this condition.
The twenty-six bones of the foot are held in place by numerous muscles and ligaments. It is impossible to give these muscles proper exercise and to keep the feet in healthy condition, if the shoe is too narrow or too pointed. A hygienic shoe should be everywhere as wide as the sole of the foot, and wide enough in front to permit the toes to move freely. The inner edge of the shoe should be straight, so that a line drawn back from the middle of the great toe touches the heel. The heels should be low and broad. The soles and uppers should be flexible, so that the foot may be bent freely. A high shoe should not be so tightly laced at the top as to interfere with circulation. A porous shoe, like one made of russet leather, h much better than an enamel or patent leather shoe, because it allows the escape of moisture and prevents overheating of the foot.
Corns are usually due to the wearing of shoes that are too tight or of a wrong shape, or to faulty posture. It is much wiser to wear shoes of the proper size than to suffer the pain which narrow shoes may cause.
Injuries to the Bony System.Sometimes when a person twists or strains some part of the body too violently, the ligaments which join the bones are torn loose. This is called a sprain. If the twist is so severe as not only to break the ligaments but to pull the joints of the bones out of place, it is called a dislocation. When a person has a bad fall or other accident, the bones themselves may be broken. Both ligaments and broken bones will in time knit together and heal, but in order that this may occur it is important that a dislocated or broken bone should be put in its proper position by a physician as promptly as possible after the accident.
QUESTIONS FOR DISCUSSION AND REVIEW
1. Of what two substances are the bones composed? What quality does each substance give?
2. Some of the larger bones are not solid; what advantage is gained?
3. Why should a baby be prevented from walking too soon?
4. A boy and his grandfather were in an accident in which they were thrown from a carriage. They were thrown in about the same way, but the man had several bones broken and the boy was merely bruised. How do you account for this? If the boy and his grandfather had each broken an arm, which would probably have recovered sooner?
5. Describe the principal parts of the skeleton.
6. How many different-shaped bones can you locate? Tell how each is especially adapted to its work.
7. How many hinge joints can you locate? How many ball-and-socket joints?
8. Why should you think that the shoulder joint would be more easily dislocated than the knee joint?
9. What would be the disadvantage of having a hinge joint at the wrist?
10. How are the different parts of the spinal column held together?
11. Explain how the spinal column bends.
12. What advantages are there in holding the body well?
13. What are some of the deformities that may come from bad habits in standing and sitting?
14. James has a desk that is too high for him; Mary’s is too low. What bad deformity may come to both James and Mary, as a result?
15. Make a study of the heights of seats and desks in your schoolroom, noting whether they are arranged so that the children can sit comfortably without twisting or bending over.
16. Charles has been carrying his books in his left arm, so as to leave his right arm free. Jane told him to divide them, or to change them frequently. Explain why Jane’s advice was good.
17. After your bath, step with wet feet on the floor and ex-amine your footprints. Are they curved and irregular or nearly straight in outline? Which should they be?
18. Why is it said that soldiers are only as strong as their feet?
19. Describe a shoe which is ideal from a hygienic standpoint.
20. What danger is there in narrow, pointed shoes?
21. What is a sprain? A dislocation?