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The Human Body:
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Skeleton
There are 206 bones in the skeleton and all of which are well connected and form the framework of the body, giving it shape. Otherwise, one will be like a deflated balloon and unable to move. One moves not only because of the skeleton but also because of the muscles and tendons which are ultimately controlled by the nervous system. The skeleton also served to protect the delicate organs and tissues. The skull shields the brain, the spinal column protects the spinal cord etc.
Bones are very strong but are very light. Each of which has a hard, compact outer layer made from a fine network of bone threads like a honeycomb, and is filled with bone marrow. Bones also contains collagen which gives them strength and prevents breakage. The external protective layer is known as the periosteum, which play an important role in building up new bone.
Bones come in all shapes and sizes. There are 4 main types. Firstly, the long bones like the humerus in the arm which has a cap-like shape at the ends. The second type is the short bones, like those of the wrist which are as wide as their length. It allows them to resist the pressure caused by the movements of the person. Thirdly, are the flat bones. There are 2 outer layers of hard bone with a spongy layer in between to help protect body organs. They also provide large surface areas like the shoulder blades. Lastly, they are the irregular bones which are found in places where extra strength and flexibility is required like the vertebrae of the spine.
Bones are made up of 2 kinds of matter. The first is 30% living tissue and the second, 70% mineral. The bones are made of coarse fibers which can grow and repair themselves. The fibers contains mainly calcium and phosphorus.
The places where bones meet is known as joints. Different joints allow different movements, some more restricted than others. The ball and socket joint is found where the arm connects to the shoulder and is the most flexible. The hinge joints are found in the knee and it only allows 1 kind of movement.
Bones are very soft while the baby is in the womb and only begins to harden with time. However, even when one is born, bones are still soft because of the huge amounts of cartilage. Cartilage is tough and gristly not like bones. The hardening of the bones is a process known as ossification and is continuous. By the age of 25, all the bones should have hardened and height becomes permanent. But some cartilage, like that of the nose, never hardens. Bones start to harden in ossification centres. They are in long bones, bone shaft and the bone cap at each end. From these starting points, cartilage is replaced by hard bones. The bones develops and change with growth. There are growth plates made of cartilage below the bone caps at the ends of the bones whose cells can multiply and so, grow till the hardening process occurs.
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