The Structural System
True. One of the primary functions of the muscles is movement.
Movement is also made possible by the joints. Joints refer to the regions of the body where two or more bones come together, usually for the purpose of motion. Some joints, like those in the cranium, or skull, allow very little movement; while other joints, like the elbow and knee, allow a great deal of movement. Ball and socket joints, like the shoulder and hip, allow movement in many different directions. Hinge joints, like the elbow, knee, and finger joints, allow for movement in one plane only.
The purpose of joints is to allow the movement of bones. But bones and joints can not move themselves. They are moved by the contractions of muscles which attach to the bones and cross the joints. Excessive movement, which could dislocate a joint, is prevented by ligamentsstrong elastic connective tissues that cross the joint. If a joint is forcefully extended beyond its normal range of motion one or more ligaments can tear resulting in a sprain.
In freely movable joints, the areas of bone that rub against each other are covered with a smooth cartilage and are lubricated by synovial fluid. These joints are known as snovial joints. The synovial fluid is contained in a capsule, formed by ligaments, which surrounds the joint. The synovial fluid is secreted by special cells which lie on the inside of the capsule. Synovial fluid is also produced in membranous sacs, known as bursae, which cushion areas where muscles and tendons rub against bones. When bursae become inflammed due to overuse or misalignment, a painful condition known as bursitis (-itis = inflammation) can result.
Excessive movement, which could dislocate a joint, is prevented by strong elastic connective tissues that cross the joint and are known as ___________.
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