Category : 6th Class
A place where two or more bones join is called a joint. Bones are joined by tough, flexible bands of elastic tissue called ligaments. A layer of smooth, rubbery tissue called cartilage, covers the surface of a bone where it joins another. This reduced friction between the surfaces when the bones move against each other. The freely moving joints are also surrounded by a fluid- filled capsule. The fluid, called synovial fluid, helps movement the way a lubricant (oil) helps movement of machine parts.
Broadly speaking, joints are of two types—fixed and movable.
Some joints, such as between the bones of the cranium, do not allow any movement. These are fixed joints. The joints between the teeth and the jaw bones are also fixed, as also are the joints between the bones that make up the hip bones.
There are many movable joints in our body. Let us study about them in some detail. Ball-and-Socket Joints: These joints allow the greatest freedom of movement or are the most mobile. In such joints, the rounded head of one bone fits into the hollow, cup-shaped socket of another. The bone that fits into the socket is free to move in all directions about the joint. The joint of the upper arm and the shoulder bone is an example of this type of joint. That is how a bowler can swing his arm or a swimmer can rotate his arms freely. The hip joint too is a ball-and-socket joint.
Hinge Joints: These joints work like a door hinge or a penknife. In this type of joints the convex (slightly bulging) surface of one bone fits into the concave (slightly depressed) surface of another bone, allowing movement only in one direction. For example, the hinge joint at your elbow allows you to bend your forearm and straighten it. It does not let your bend your arm backwards or sideways. The same is true of the knee joint.
Pivot Joints: In such joints, a bony ring rotates around a pivot (axis), or stick like bone. This allows twisting movement about the joint. The head moves from side to side because it rests on a pivot joint at the top of our spine.
Gliding Joints: Such joints occur between the vertebrae and between the bones of the wrists and ankles. The cartilage between these bones allows small movements, somewhat like the movement of a spring. These are not freely moving joints..
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