The 206 bones of the body make up its skeleton. Each bone forms a hard, rigid inner support for its part of the body, and anchorage points for muscles to pull as the body moves. The old bones of a museum skeleton are dry, brittle and crumbly. But inside the body, a bone is a very active, living part. It is not dry – it contains about one-fifth water. It is not brittle – it is slightly bendy because it contains fibres of a flexible substance, the body protein called collagen. Bone is also very tough because it contains hard crystals of minerals such as calcium phosphate. And like any other body part; a bone has a supply of blood vessels and nerves.
Most bones are not solid. They are hollow, with a cavity inside. This contains a soft, jelly-like substance called bone marrow. The marrow makes new microscopic red and white cells for the blood at the rate of two million every second, to replace the old, dead blood cells.
TYPES OF JOINTS
Bones are linked together at joints. In some joints the bones are fixed or cemented firmly to each other and cannot move, as with the small bones of the face. These are known as suture joints. In other joints the bones can move in relation to each other. The body has many different kinds of moveable joints, depending on the shapes of the bone ends and how they fit together. They resemble the joints used in machines to give a certain kind of movement. For example, the knee is a hinge joint and moves only backwards and forwards. The hip is a ball-and-socket joint and can move in any direction.
INSIDE A JOINT
In a moveable joint the ends of the bones are covered with a smooth, shiny, slippery, slightly soft substance called cartilage. This prevents the bone ends rubbing against each other and wearing away. The cartilage surfaces slip over each other with hardly any wear. A flexible bag, the synovial capsule, wraps around the bone ends. This makes an oily fluid which fills the bag and lubricates the joint, like the oil in a machine. The two bones are linked by flexible, strap-like ligaments around the synovial capsule. These stop the bones coming apart and prevent the joint bending too much.
FRACTURES
Bones are strong but sometimes they cannot withstand the stress put on them, especially in an accident. A bone may crack or snap. This is known as a fracture. In a compound fracture, the broken ends protrude through the skin. Part of the bone shatters into small parts in a comminuted fracture. The parts of the bone ram into each other in an impacted fracture.
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