The main functions of the skeleton are to protect soft organs, support soft tissues and provide movement through the muscles attached. Main parts of the skeleton are: skull (cranium), jaw (mandible), rib, backbone, breast bone (sternum), upper arm (humerus, lower arm (ulna and radius), thigh bone (femur), kneecap (patella), leg bones (fibula and tibia).
Bones
The human skeleton contains 206 bones to provide protection and movement. Delicate organs like the heart and the lungs are protected by the rib cage. To provide protection, bones need to be strong and hard to withstand pressure and hard blows. However, they also need to be light in weight to make them easy to move.
To be strong and light weight, larger bones are made up of hard dense substance but are hollow in the middle, the hollow part is filled with bone marrow which produce red blood cells.
Movement
Bones are connected together by a joint which allows movement. Bones in a joint are held together by strong fibres called ligaments, and movement at joints is controlled by muscles. Each end of a muscle is connected to a bone by a tendon which will not stretch but allow the muscles to pull on the bone. One end of the muscle is attached to the bone which does not move and the other is fixed to the bone that moves. The muscle pulls on the bone by contracting.
The humerus (upper arm) has two muscles, biceps and triceps. Arm bending is by contracting the biceps with the triceps relaxes, the contraction of the biceps pulls on the bone to bend the arm. To straighten the arm, the biceps relaxes and the contraction of the triceps pull on the bone to straighten the arm. Muscles over the body are arranged in pairs to work in opposite directions and they are known as antagonistic muscle pairs.
Joints
Most joints in the body are synovial joints that allow considerable freedom. A synovial joint is enclosed by a tough capsule containing synovial fluid, a very good lubricant, and the surfaces of the bones in contact are covered with a smooth layer of elastic cartilage. These features reduce friction between the bones to allow bones to slide easily over each other. There are two types of synovial joint, hinge joint (e.g. knee, elbow, wrist) and ball and socket joint (e.g. hip).