Many of our bones are joined to one another through joints, which permit different kinds of movements. The ball joints of our hips and shoulders offer the highest freedom of movement. Here, the ball-shaped end of one bone moves inside the bowl-shaped end of the other. Knees and elbows are joined by means of a hinge joint, which permits only the back and forth movement, like the hinge in a door. Our head sits on a condyloid joint, which allows it to move sideways, front or back, but not rotate. Fluid is present in the joints as a lubricant so that the joints and the bones do not rub against each other.
