Framework of the body
Joints: the skeleton in action Wherev~r
one bone meets another in the human bod y there is a joint of some kind. These linkages are of various designs, each one custom-built to fulfill a particular function. Some joints, for example those between the bony plates of the skull, do not move at all. At the other extreme are highly versatile joints like those in the hip and in the shoulder masterpieces of engineering which give a phenomenal range of power and movement. Freely movable joints are especially complex in structure because of the demands made by mobility. Where two bones meet and move there is friction and if, like the hip or knee, the joint is also load-bearing, there is the additional stress caused by such everyday movements as walking or climbing stairs. At the joint, therefore, each bone is capped with cartilage, which reduces friction and acts as a buffer. To offset friction, the movable joint is contained in a tough, fibrous capsule lined with
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synovial membrane. This membrane secretes \\ At the elbow an d knee, hinge joints combine a runny, yellow-tmged substance, synovial strength with mobility. Here is a design yieldfluid, which lubricates the working parts. All ing only bend -stretch movements, although bony surfaces, tendons and ligaments involved the knee assembly, with crossed ligaments in in the joint are covered with synovial 111em- the center uniting the femur and tibia, is somebrane, which extends to the edges of the carti- what more complex than the elbow. Besides lage layers. Beyond the capsule are ligaments, the p rimary hinge movement between the which bind and strengthen the joint, and the upper and lower parts of the leg, the knee also tendons of muscles, which are necessary to contains a gliding mechanism where the patelmove its bones. Ia, the kneecap, slides over the femur. In large joints, like the knee, there are small, At the elbow, giving greater mobility to the fluid-filled sacs, known as bursas, which act as strength of the associated hinge joint, is a shock absorbers. Also lined with synovial . pivot joint which enables the two bones of the membrane, bursas are found between the mov- forearm, the radius and the ulna, to twist. able parts (where bone meets tendon, muscle The shoulders and hips are both served by or ligament) lying outside the capsule. Some- the ball-and-socket-type joint- a freely movtimes undue stress on a joint causes the bursa ingjoint which gives a wide range of play to the to produce excessive fluid , giving rise to in- limbs. At the hip, the head of the femur fits into Ram mat ion, pain and stiffness. This is a con- a deep socket, called th e acetabulum, where the dition known as bursitis, and commonly c¡a lled ilium, ischium and pubis meet. A rim of carti" tenniselbow"or"housemaid'sknee." lage helps to grip the ball firmly, and strong
M ove m ent at the j oints ellipsoid joi nt
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boll- end-socket joints './;ftJJ:..tJj-~
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A hi nge joint at the elbow allows the arm to bend and extend. The condylar joint at the knee gives similar movement plus some rot ation. Mobile ellipsoid join'ts between t he fingers and palm of the hand allow circular movements but no rotation. Ball -and-socket joints, like the one at the shoulder, g ive maximum freedom of movement. At t he other extreme, plano joints between the toe bones permil only a small degree of gliding movement and the pivot joint bet ween the two cervical vertebraethe atlas and the axi s- merely provides rotation of the head. The saddle join! at the ankle i s almost as mobile as the shoulder joint, bul rotation is far more limited .