Now think of the head and its movements on the neck. We are able to move the head forward and backward, as in the act of nodding, and to depress it towards either shoulder. This, you will readily see, is the work of a double hinge-joint. The joint is between the base of the skull andthe top-most vertebra on which it rests.
But we have only to turn the head right and left, to be reminded that there is another movement provided for,besides that of the hinge. In the top-most vertebra which carries the skull there is a small hollow ring, into which fits an upright peg or pivot of the second vertebra. When the head turns right and left, it is this top-most vertebra which really moves round on the peg of the second, and in moving it carries the skull round with it.
Such a joint is known as a pivot-joint.
There is one very important arrangement that must not be lost sight of in connection with all these joints. The joints are designed for movement, and all movementsmust be smooth and easy. There must be no friction, no grating.
