| Definition:
Joints, aka articulations, are the sites where bones
meet one another. |
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 Structure:
Joints are classified into three structural types; fibrous, cartilaginous,
and synovial. Examples for each are the skull, the pubis and the shoulder
joint. Fibrous joints are joined by collagen, cartilaginous joints unite
bones with cartilage, and synovial joints are enclosed by a strong, fibrous
capsule. The capsule is continuous with cartilage that covers and protects
the ends of the bones. Inside the synovial joints, a fluid, called synovial
fluid, is secreted for lubricating the joint. Ligaments are special thickened
areas of fibrous tissue around, or through, the joints, making them strong,
and determining the flexibility of the joint.
 Function:
Immobile joints hold bony structures together,but mobile joints permit
bones to move. The four types of mobile joints are: hinge joints, like
the elbow or knee,
that allow movement back and forth; ball-and-socket joints, like the hip
and the shoulder, that allow a
large range of motion; pivot joints, like the vertebra or the elbow,
that allow turning of a bone around itsÕ own axis; and gliding joints,
like the rounds bones of the wrist or ankle,
that permit slipping of two bones in relation to each other.
Synovial joints are especially important in locomotion
because they allow free movement. The six types of synovial joints, with
examples of each, are: ¥plane/tarsal & metatarsals, ¥hinge/elbow
& knee, ¥pivot/radius & ulna, ¥condyloid/metatarsal
& proximal phalanx, ¥saddle/carpal & metacarpal, and
¥ball and socket/ilium & femur. Other joints, for example,
the joint of the jaw bone, important for eating,
and the pivot joints of the vertebra,
allow for keeping stability, as for posture.
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