What is Joints
Joints are locations where two or more bones come together, or articulate. Bones are joined with varying degrees of rigidity. Joints may be fixed, as in the skull, semimovable, as in the vertebral column, or movable, such as are found in the limbs. The skull bones meet at sutures, wavy lines where they form interlocking projections that hold them together. The shape of movable joints in the human skeleton can vary with function so that they are remarkably similar in structure and function to those you would find in a mechanical robot. There are ball-and-socket joints, hinge joints, pivot joints, saddle joints, ellipsoid joints, and sliding joints.
The ends of the bones involved in movement are covered by an smooth, plastic-like elastic sheet of connective tissue. The whole joint is surrounded by a sac, called the synovium. Fluid in the sac, the synovial fluid, lubricates the joint. The fluid-filled sac of a knee or shoulder joint is called a bursa. When these joints are injured, the pain and inflammation is called bursitis. Tearing or overstretching of ligaments is called a sprain.
Tendons, ligaments, and skeletal muscle are attached to the periosteum of bones at the joints. Tendons are connective tissue that attach muscles to bones. Ligaments are composed of tough bands of connective tissue, and hold together movable joints, connecting bone to bone. Skeletal muscles are responsible for bone movement, and are discussed further in the next section.