Joints

Joints are essential for every kind of movement that involves the bony parts of the body, and locomotion is no exception. A joint is a point of contact between two bones, or between a bone and a cartilage.

The force needed for movement is produced by the muscles, but that force is actually applied through the joints. Here the joint acts as a fulcrum — the fixed point about which the bony lever turns. How much movement a joint allows is not the same everywhere; it varies depending on several factors, mainly the structure of the joint.

On the basis of their structure, joints are grouped into three major forms: fibrous, cartilaginous and synovial.

Fibrous joints

At a fibrous joint the two bones are clamped together by a tough, dense band of fibrous connective tissue, leaving no room for any movement between them at all.

• Example: the seams between the flat bones of the skull -- the sutures that knit the cranium into a single rigid case -- are the clearest example.

Cartilaginous joints

A cartilaginous joint uses a pad of cartilage, rather than fibrous tissue, to link the two bones -- a softer connection that allows only a modest, restricted range of movement.

• Example: the junctions between one vertebra and the next, all along the vertebral column, work this way.

Synovial joints

A synovial joint is built quite differently: a fluid-filled pocket, the synovial cavity, separates the two bones' contact surfaces and lets them glide against one another with comparatively little friction. That fluid cushion is what makes a synovial joint capable of considerable movement -- and it is this type, more than the other two, that locomotion actually depends on.

There are several kinds of synovial joint, each named for the way its surfaces fit and move: …