The skeleton, or bony framework, of the body is very important. It acts somewhat like the framework of a house in that it gives the body shape and support. Some of the bones also protect delicate organs, and some work with the muscles in giving the body strength. Altogether the skeleton consists of more than two hundred separate bones. These bones are not fastened closely together, except in the skull, but are hinged so that the body may bend and take different positions.
What the Framework Is Like
The spinal column. The spinal column is a pliable chain of bones extending down the center of the back. It serves as a support to which the ribs are fastened and thus helps to provide a safe place for the heart, lungs, stomach, and other vital organs. The ribs are flat bones that curve around in front, giving shape to the body. Some of them are fastened to a broad, flat bone at the front known as the sternum, or breastbone. The chief bones of the shoulder are the scapula, or shoulder blade, and the clavicle, or collar bone. These bones help to give shape to the shoulders and help to support the arms. Near the lower end of the spinal column are fastened broad bones known as the pelvic bones, which help to support the intestines and other vital organs. Two sockets in these bones hold the upper bones of the legs in place. The pelvic bones are very important, as they give the body both protection and support. The spinal column, then, with all the bones attached, except those of the head and the limbs, gives shape and support to the part of the body known as the trunk.
Altogether the spinal column consists of twenty-six separate bones. They are odd-shaped but exceedingly strong. Moreover, they are much more useful to the body as twenty-six separate pieces than they would be if they were one long, solid bone. They allow the body to bend much more readily than it otherwise would. They also act somewhat like a spring, protecting the body from jars. One of the greatest uses of the spinal column is to provide a safe covering for the spinal cord, a large bundle of nerves that lead from the brain.
The upper part of the arm contains one bone, the humerus, while the lower part contains two bones, the ulna and the radius. The wrist contains eight bones, the palm of the hand, five, and the fingers, fourteen. The twenty-seven bones of the hand and wrist together make up one of the finest and useful tools known. No other part of the body of any living thing can do such a variety of things.
The legs and feet have the same general pattern as the arms and hands, but are larger because they must carry the weight of the body. It hardly ever happens that a person’s body becomes so heavy that the legs cannot support it. The upper part of the leg contains one bone, the femur, and the lower part of the leg contains two, the tibia and the fibula. Each foot, like the hand, is made up of twenty-seven bones. Although it is little used as a tool, it can be trained to do many things usually done with the hand. Sometimes people who lose their hands learn to make great use of their feet. Altogether the bones of the legs and feet work together in a wonderful way to enable us to move about.
Bones of the head.
The bones of the rounded part of the head are known as the cranium. Their chief purpose is to protect the brain and other delicate organs such as the eyes and ears. The eyes are set in sockets in the front of the cranium and the ears in openings at the side. The bones of the cranium also help to protect the delicate organs of taste and smell, which are located in the mouth and nose.
The cranium is of such a shape that it is not easily injured. Although it is made up of separate bones, they are so closely knitted together that they seem like one. Moreover, they are very strong, many of them being hollow or built with a framework inside to give them strength. Thus the cranium is especially well built to give protection to the brain and other delicate parts.
Structure and Use
How bones are made.
The bones of the body are of different shapes and sizes according to the work they do. The shoulder blade, or scapula, is fiat and broad, and the humerus of the upper part of the arm is straight and long. The bones of the cranium are somewhat curved, while those of the hands and feet are small and huddled together.
The bones also vary greatly in their structure, which means in the way they are made. Some bones are hollow. The hollow part of a bone is known as a sinus. There is a sinus, for example, in the cranium above each eye and another on each side of the nose in the bones of the face. These sinuses sometimes become very seriously infected and cause a great deal of trouble. Some bones, as the femur in the upper part of the leg, are only partly hollow. This bone must be very strong to hold the weight of the body. Inside the bone is a cavity crossed by countless fibers that form a network. These fibers help to give the bone strength.
Bones contain blood vessels and nerves just as do other parts of the body. Most of these blood vessels and nerves are found on the inside of the bones. The inside of most bones is filled with a soft substance called marrow. This substance is very important, as it helps to make the blood and to furnish food for the bones. It is important that food contain minerals to make the bones hard. Milk is especially good for the purpose since it supplies the minerals that are needed.
How the bones are fastened together.
The bones and muscles would be able to do. little work were it not for the movable joints which fasten the bones together. In fact, the body would be stiff like the trunk of a tree. Perhaps you will better understand the need for joints if you imagine trying to use your arm without an elbow. There are four kinds of joints in the body. A hinge Joint, as that at· the elbow, allows movement in two directions. A ball-and-socket joint, as that in the shoulder, permits movement in circles. A pivot joint, as that at the top of the spinal column, permits one part of the body to turn upon another. A sliding joint, as that in the wrists, permits a great variety of movements.
All parts of movable joints are lined with a very thick membrane. This membrane serves as a pad, or cushion; between the bones. It also provides a smooth place for one bone to work upon another. An oil is always secreted to keep the membrane soft and pliable. The bones forming a joint are held in place by tough bands called ligaments. If these ligaments are torn from their fastenings, the injury may be difficult to repair.
How the muscles and bones work together.
The muscles are able to move the body because they use the bones as levers. A lever is a tool, like a rod or pole, which is used to make work easier. Many examples of levers may be seen every day. A pump handle is a lever. If you were to pull straight up on the rod in a pump, you might not be able to lift it, but when you press down on the end of the long handle you raise it quite easily. A workman often uses a long pole or iron bar to move a heavy weight. Likewise, the muscles use the bones in such a way as to do the most work with the least possible strain. If it were not for the use of the bones as levers, people would have far less strength than they have.