Joints

A joint is any point of contact between two bones, or between a bone and a piece of cartilage, and joints are what allow the otherwise rigid bones of the skeleton to move relative to one another under the pull of skeletal muscles. Structurally, joints are classified into three broad classes according to how much movement they permit.

Fibrous joints are immovable joints in which the adjoining bones are held tightly together by a thin layer of fibrous connective tissue, with no cavity or cartilage between them and no movement possible. The classic example is a suture, the interlocking, tooth-like joint found between the flat bones of the skull, which becomes progressively more rigidly fixed as the skull matures.

Cartilaginous joints are slightly movable joints in which the adjoining bones are connected by a pad or disc of cartilage rather than by direct bony contact, permitting a small amount of bending or compression but no free rotation. Examples include the symphysis pubis, the slightly movable joint between the two pubic bones at the front of the pelvis (which can loosen further during childbirth), and the joints between the bodies (centra) of adjacent vertebrae, where intervertebral discs of cartilage permit the small movements, added up along the whole column, that give the back its overall flexibility. …