Why do we have bones?

Bones - we have bones to give us shape and support the body, to protect and contain the body's delicate vital organs, and to help us move about. Bones are made from living tissue, composed of special cells which secrete around themselves hard material rich in calcium salts. In a child the bones are soft and cartilaginous (Cartilage is gristle, the elastic substance of the ear or nose). Bone making, or ossification, is a gradual process. A baby has as many as 270 bones, but an adult only 206, for some bones grow together as they get older. All bones have a middle cavity filled with a yellow or red fatty substance called bone marrow, the blood making factory of the body, which also keeps bones light without reducing their strength. The basic part of the skeleton is the spine, which has 33 bones or vertebrae. The spine carries the weight of the body, is extremely flexible and contains and protects the delicate spinal cord. The skull shelters the brain while the ribs protect the heart and lungs. Bones fit together at the joints and are held firm by ligaments, which are made of tough tissue like cords or straps. Inside each joint is a thin membraneous bag which secretes a lubricant to make the joint move smoothly. Some glide on one another, as the lower jaw slides on the upper. Some, such as the elbow and the knee, hinge on each other and others, like the hip, make a ball and socket joint. Bones are sometimes fused together and immovable, as in the five large, lower vertebrae called the sacrum.

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