Whenever two bones glide over another, a joint exists. A joint allows the bones to glide smoothly with very little friction. This is because the ends of bones are covered by cartilage so that they don’t really rub against each other. In addition, a whitish fluid which has the consistency of egg white is secreted by the joint. This is called synovial fluid which acts like oil in a machine to reduce friction.
In the human body, there are four important types of joints. One of them is a ball-and-socket joint such as is found at the shoulder. This joint has the greatest range of motion which enables us to move our arm in any direction in space. The hip joint is the largest ball-and-socket joint in the body. It is fitted more tightly hence it does not permit much range of motion.
The second joint is ellipsoid joint. The wrist joint is and example of this kind. It enables to have elliptical movements rather than circular ones. Saddle joint which is in our backbone, enables us to have two movements.
A third type of joint is the hinge joint. Here the bones allow us to have only to and fro movements. The joints between the bones of our fingers are hinge joints.
Last kind of joint is rotatory joint. This enables the bones to turn like corkscrew. We have a rotatory joint at the base of our skull so we can turn our heads and others in our elbows.
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