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A baby is born with
as many as 270 small, rather sot bones in his framework. A fully grown person usually
has 206, because some bones become fused, or grow together. Bones it together
at joints and are held fast by ligaments, which are like tough cords or straps.
Some joints can be moved freely. For example, when you run, you move your legs
at the hip and knee joints.
When you throw a
ball, you move your arm at the shoulder and elbow joints. Moreover, some joints
cannot be moved at all. At the base of the spine the bones are fused, forming
one bony plate that fits into another.
Neither move, the
joints in your skull are very solid, too, except for those in the jaws. The protection
that the skeleton provides includes the hard, bony cap o the skull. This
protects the brain. The rib cage protects the heart and lungs. And the backbone,
or hollow spinal column, protects the spinal cord, the body trunk line of nerve
cables. The backbone is actually a string of small bones.
It is very hard for
us to think of bone as living tissue, but it is. It grows when a person is
young. For example the thigh bone may triple in length between the time a
person is born and the time he is fully grown.
Bones grows in
length and thickness as calcium and other minerals are added to them. And since
bone is living tissue, it must be fed. The outside of the bone is covered with
a thin, tough skin. The skin holds many tiny blood vessels that carry food to
the bone cells.
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A human bone grows and
changes over the whole life. Furthermore, a healthy skeleton needs daily
exercise, walking, jumping, skipping; support the human bones to grow strong.
Also, a healthy bone mass in the skeleton reaches utmost density around the age
of thirty. Our lungs can collapse without rib cage, and intercostals muscle.
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