The muscular and skeletal systems

Joints

Joints are the areas where two bones meet. Most joints are mobile, allowing the bones to move. Joints consist of the following:

cartilage – at the joint, the bones are covered with cartilage (a connective tissue), which is made up of cell and fibres and is wear resistance. Cartilage helps reduce the friction of movement.
synovial membrane – a tissue called the synovial membrane lines the joint and seals it into a joint capsule.
synovial fluid – a clear, sticky fluid secreted by the synovial membrane.
ligaments – strong ligaments (tough, elastic bands of connective tissue) on each side of a joint attached to muscles that control movement of the joint.
bursas – fluid-filled sacs, called bursas, between bones, ligaments, or other adjacent structures help cushion the friction of a joint.

Ball and socket joints, like your hip and shoulder joints, are the most mobile type of joint in the human body. They allow you to swing your arms and legs in many different directions.
Ellipsoidal (condyloid) joints, such as the joint at the base of the index finger, allow bending and extending, rocking from side to side, but rotation is limited.
Saddle joints, these can only be found in thumbs. The bones of the saddle joint can rock back and forth and from side to side, but they have limited rotation.
Planar (gliding) joints occur between surfaces of two flat bones that are held together by ligaments. Some of the bones in your wrists and ankles move by gliding against each other.
Hinge joints, like in your knee and elbow, enable movement similar to the opening and closing of a hinged door.
Pivot joints, allow rotation around an axis. The neck and forearms have pivot joints. In the neck the occipital bone spins over the top of the axis. In the forearms the radius and ulna twist around each other.

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