These joints move each time we chew, talk or even swallow. The TMJ is actually a sliding joint and not a ball-and-socket like the shoulder. This sliding allows for pressures placed on the joint to be distributed throughout the joint and not just in one area. The TMJ is the most complex joint in the human body.
Placed between these two bones is a disc, just like the one between your back bones. This disc is primarily made of cartilage and in the TMJ acts like a third bone. The disc, being attached to a muscle, actually moves with certain movements of the TMJ.